Abstract: With the production and use of long-chain per- and polyfluoroalkyl substances (PFAS) being restricted, short- and medium-chain as well as new alternative PFAS have been widely detected in water environments, and their ecological and health risks have drawn urgent attention. However, the current detection standard system is still not perfect. Traditional solid-phase extraction (SPE) technology has the advantages of a low detection limit and strong anti-interference ability, but it has disadvantages such as long-time consumption, high cost and easy contamination in practical application. Here, a method for the determination of 18 trace short- and medium-chain (ng/L level) and new PFAS in environmental water, including 7 perfluorocarboxylic acids, 4 perfluorosulfonic acids and 7 new alternatives, by ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was established by direct injection after filtration. The effects of sample protectants, filter membranes, injection solvents and injection bottle materials were systematically investigated. The results showed that the addition of 10% methanol could significantly improve the stability of the sample. The adsorption behavior of five filter membranes (regenerated cellulose, polyethersulfone, polytetrafluoroethylene, mixed cellulose and nylon) in 4 solvent systems was studied. It was found that the nylon membrane had the highest adsorption rate (above 90%) in pure water and 10% methanol-water solution system, while the regenerated cellulose membrane had the lowest adsorption rate, making it the best filter membrane for this method. For the large volume injection of this method (50μL), 10% methanol-water solution was the ideal injection solvent, and polypropylene injection bottles had the best storage effect. This method has the following advantages: (1) It is simple and fast, with direct injection after filtration, no solid-phase extraction and concentration process, and the sample pretreatment time for a single sample is less than 5min, which has more than 10 times the efficiency of the traditional SPE pretreatment method; (2) It is low-cost, reducing solvent consumption and the use of solid-phase extraction columns and other consumables; (3) Since the sample pretreatment only involves filtration, it not only reduces the possible loss links but also significantly reduces the risk of contamination. This method can be applied to the rapid monitoring of trace PFAS in environmental water, especially suitable for large-scale environmentally targeted screening and emergency monitoring scenarios. However, for PFAS samples with concentrations lower than 0.7ng/L, the quantitative accuracy is challenged due to the limitation of instrument sensitivity.