Okadaic acid （OA） is a fatty acid polyether biotoxin that poses risks to human health and the ecological environment. Determining the concentrations of OA in seawater not only allows for early warnings of potential toxin accumulation in marine organisms， but also helps clarify the actual impacts of OA on marine ecosystems. However， OA exists in seawater at extremely low concentrations， and the matrix has a high salt content. Thus， adequate sample pretreatment is necessary prior to high performance liquid chromatography-tandem mass spectrometry （HPLC-MS/MS） analysis. Film-based solid-phase extraction （F-SPE） has attracted considerable attention in sample pretreatment. Unlike conventional solid-phase extraction （SPE）， which relies on large quantities of adsorbent particles， F-SPE uses a thin film， thereby addressing common issues in SPE applications such as high column pressure， column clogging， and adsorbent leakage. Currently， most such films are prepared by modifying polymer film skeletons with a certain amount of adsorbent particles. Nonetheless， these adsorbent particles have inherent limitations， including insufficient adsorption capacity or irreversible adsorption， leading to poor extraction performance for specific targets. Therefore， developing new types of films with high extraction efficiency is highly significant， as it can facilitate the wider application and popularization of this technology. Covalent organic frameworks （COFs） are porous crystalline materials with adjustable pore size， high specific surface area， and excellent stability. COF films can not only inherit the high permeability of films， but also possess strong COF adsorption capacity. In this work， a COF film （TPB-BTCA） was synthesized through the liquid-liquid interface synthesis method under mild reaction conditions， and characterized in detail. The Fourier-transform infrared （FT-IR） spectrum， X-ray diffraction （XRD） pattern， and X-ray photoelectron （XPS） spectrum verify the successful preparation of the TPB-BTCA film using a Schiff base reaction. The scanning electron microscopy （SEM） images and water contact angle testing show the prepared TPB-BTCA film is a hydrophilic heterogeneous film with specific morphologies， which is beneficial to a more thorough and rapid contact between the film and the sample solution. The nitrogen adsorption-desorption experiment demonstrates the TPB-BTCA film has a high surface area （1 261.6 m²/g） and porosity （0.6 cm³/g）， providing sufficient and easily accessible sites to adsorb these OA molecules. Subsequently， the TPB-BTCA film-based F-SPE method was used for the extraction of OA in seawater. At the same time， the influence of potential factors on the extraction efficiency was also investigated， including the sample loading volume， the pumping speed for loading， the eluent type， the volume of the eluent， the pumping speed for elution， and the salt concentration. Under the optimal conditions， the TPB-BTCA film exhibited excellent extraction performance for OA. Finally， a new method for the detection and analysis of OA was established by combining F-SPE technology with HPLC-MS/MS instrument. The developed method has a wide linear range （0.8–500.0 pg/mL） with good linearity （r=0.999 0）， low limit of detection （0.2 pg/mL） and satisfactory precision （RSDs≤6.4%， n=5）. Meanwhile， seven seawater samples were used to evaluate the feasibility of the developed method for detecting OA in actual samples. Ultra-trace amounts of OA were detected in two seawater samples with concentrations of 5.4 pg/mL and 61.8 pg/mL， respectively. Compared with the reported HPLC-MS/MS methods， the developed method only requires a single layer of film to process large-volume （100 mL） seawater samples and achieve the lowest detection limit. This may be due to the advantages of the TPB-BTCA film， such as high permeability， high porosity， and high specific surface area， which enable it to efficiently extract OA from seawater. In conclusion， this study not only provides an effective analytical method for detecting ultra-trace OA in seawater， but also demonstrates the application potential of COF films in sample pretreatment.