In this study， an online solid-phase extraction-ultra performance liquid chromatography-tandem mass spectrometry （online SPE-UPLC-MS/MS） method was established to rapidly screen and determine 51 per- and polyfluoroalkyl substances （PFASs） in raw and drinking water. Ammonium formate and 24 PFAS internal standards were added to each sample. The ammonium formate concentration in the sample was 2 mmol/L after mixing， and the contents of the PFAS internal standards ranged between 2.5 and 50 ng/L. Each sample was filtered through a 0.22-μm cellulose acetate filter， and a 5-mL aliquot was injected and adsorbed using an HLB online solid-phase extraction column and rinsed with 2 mmol/L ammonium formate. Acetonitrile and 2 mmol/L aqueous ammonium formate were used as mobile phases. Separation was performed on a BEH C₁₈ chromatographic column with gradient elution. Electrospray ionization source negative ion mode and multiple reaction monitoring mode were used for detection， with quantification performed using the internal standard method. The method was validated by determining accuracies and precisions for the 51 PFASs using raw water and drinking water as matrices. Excellent linear relationships within their respective ranges were observed， with correlation coefficients （r²）>0.995. The method exhibited limits of detection （LODs， S/N=3） and quantification （LOQs， S/N=10） of 0.03–1.5 and 0.1–5.0 ng/L， respectively. The PFASs were spiked at levels of 1， 10 and 50 ng/L， with spiked recoveries of 60.2%–126.9% and 60.4%–122.6% obtained for raw- and drinking-water samples， respectively， along with corresponding relative standard deviations （RSDs， n=6） of 0.3%-17.9% and 0.4%-17.7%， respectively. The developed method was used to determine PFAS residues in raw and drinking water， with perfluoroalkyl carboxylic acids， perfluoroalkyl sulfonic acids， and perfluoroalkyl ether acids detected at relatively high rates with contents of 0.1–209.7 and 0.1–63.6 ng/L in raw water and drinking water， respectively. Compared to offline solid-phase extraction methods， the developed method requires fewer samples， is more convenient from a sample-collection perspective， and uses lower quantities of internal standards. It also analyzes rapidly and is highly sensitive and reproducible. Only 20 min was required to determine the 51 PFASs at the ng/L level， from online enrichment to detection. The developed method is suitable for the trace determination of various types of PFAS， such as perfluoroalkyl carboxylic acids， perfluoroalkyl sulfonic acids， perfluoroalkyl ether acids， fluorotelomers， and fluoroalkyl sulfonamides， in raw and drinking water， thereby effectively boosting the detection efficiency for perfluoroalkyl substances in water in a manner that has substantial practical applications value.