Abstract: Objective: To establish a gas chromatographic method for simultaneous determination of residual triisopropyl phosphite and tetraisopropyl methylenediphosphonate in methylenediphosphonic acid. Method: HP-FFAP (25 m×0.32 mm, 0.5 μm) was used as the column, the temperature was programmed, the initial temperature was 40 ℃, and it was kept for 3 minutes. The temperature was raised to 220 ℃ at 15 ℃/min for 6 minutes. The carrier gas was nitrogen, flow rate was 2.5 mL/min; inlet temperature was 200 ℃, no split injection; sample ready to use, injection volume 1.0 μL; detector was FID detector. Results: The separation of the two impurities from the adjacent peaks was good, the stability of triisopropyl phosphite was poor, and the high temperature conditions were stable only within 2 hours. The linear range of triisopropyl phosphite was 6.081~194.6 mg/L (r=0.999, n=6), the recovery of the sample was 96.04%, the RSD was 1.73%(n=9); the linear range of tetraisopropyl methylenediphosphonate was 6.272~200.7 mg/L (r=0.999, n=6); the recovery rate of the sample was 97.72%, and the RSD was 3.14%(n=9); three batches of samples were identified using the established method and the NMR method of the European Pharmacopoeia. No target impurities were detected. Conclusion: The established method is fast, simple, accurate, precise and reproducible, and can be used for quality control of two process impurities in methylenediphosphonic acid bulk drugs.