The invention disclose a new process for synthesizing acephate, using acetyl chloride as an acylating agent to react with acetyl chloride. The methamidophos almost completely translates into acephate by the process of micro-vacuum pumping, dissolve exsolution, separation and mother liquid recycle, wherein the yield and content may both reach more than 95, and the by-product is hydrochloric acid. The process is simple and safe, has low cost without three-waste and environment pollution, and has simple postprocessing.