Perfluoroalkyl and polyfluoroalkyl substances （PFAS） possess desirable properties， including hydrophobicity， oleophobicity， surface activity， and thermal and chemical stability. Their extensive production and widespread application have resulted in the pervasive presence of PFAS in diverse environmental media. However， accumulating evidence indicates that PFAS are persistent， capable of long-range transport， bioaccumulative， and toxic； consequently， their adverse effects on ecosystems and humans are of widespread concern. Aquatic environments serve as a major transport pathway and contamination route for PFAS， making accurate measurement of PFAS levels in water crucial for assessing associated environmental and health risks. However， accurate quantification requires multi-step procedures， including sample filtration， enrichment， nitrogen blow-down concentration， and reconstitution， such as solid-phase extraction （SPE） and accelerated solvent extraction （ASE）. These methods are often labor-intensive and time-consuming. Although research on fully automated SPE technology is increasing， it necessitates installation of online SPE systems， which entail high costs and may present limitations in sample throughput per run. With continuous advancements in mass spectrometry， instrumental sensitivity has improved considerably， making direct injection of water samples for multi-analyte analysis technically feasible. However， reports on the use of direct injection methods for detecting PFAS in water remain limited， and the number of target analytes covered in such studies is relatively small. In this study， a direct injection-ultra performance liquid chromatography-triple quadrupole mass spectrometry （UPLC-MS/MS） method was developed for the determination of 31 PFAS in water. To optimize the chromatographic separation， enhance the detection sensitivity of target analytes， and minimize undesirable adsorption losses， the method was meticulously optimized with respect to solvent selection， injection volume， and syringe filter type. Our method involves the following procedure： 0.5 mL of water is aliquoted， mixed with 0.5 mL of methanol spiked with 2 ng of internal standard， and filtered through a 0.22 μm polypropylene membrane. The PFAS were analyzed by UPLC-MS/MS with an injection volume of 35 µL. The analytes were ionized in electrospray ionization negative mode （ESI^-） with scheduled multiple-reaction monitoring （sMRM）. The MS parameters， including precursor and product ions， collision energy， and declustering voltage were optimized. Through optimization of the analytical column and mobile phases， the analytes were separated on an RSLC 120 C18 column with a gradient of methanol and 5 mmol/L ammonium acetate aqueous solution as the mobile phase in a gradient elution program. The results were quantified by the internal standard method. The method demonstrated excellent linearity （R²>0.994） across a defined concentration range. The limits of detection （LODs） and quantification （LOQs） were 0.007 1–3.0 ng/L and 0.024–10 ng/L， respectively. Recoveries at spiked levels of 2， 10， and 500 ng/L ranged from 67.2% to 130.2%， with relative standard deviations （RSDs） of 0.30% to 18%. To quantify the effective equivalence between the enrichment efficiency of SPE and the sensitivity of direct injection methods， a comparative analysis of analyte recovery rates was performed for both approaches. Furthermore， for long-chain PFAS， direct injection demonstrated consistent and favorable recovery performance. The method was applied to analyze PFAS in groundwater samples. The results showed that 24 PFAS were detectable with the total PFAS content （∑PFAS） ranging from 20.6 to 521 ng/L， with perfluorooctanoic acid （PFOA） and perfluorobutanoic acid （PFBA） being the primary pollutants. This approach is simple， rapid， highly sensitive， and provides broad coverage of target analytes， making it suitable for the quantitative analysis of PFAS in urban groundwater. It offers an efficient and reliable technical solution for determining trace-level PFAS in environmental water samples.