3,4,5-trimethoxyphenyl acetic acid is an important intermediate for preparation of Mivacurium Chloride, was synthesized from 3,4,5-trimethoxybenzaldehyde by dichlorocarbene reaction and reduction reaction. The optimum reaction conditions are as follows: (1)The dichlorocarbene reaction: n(3,4,5-trimethoxybenzaldehyde): n(chloroform)=1:3, sodium hydroxide solution w=40%, reaction time 7h,reaction temperature 60℃,3,4,5-trimethoxybenzaldehyde dosage 0.1mol and 2g Hexadecyl trimethyl ammonium Bromide (CTMBA)/polyvinyl alcohol-600(PEG-600)(1:1) as the phase transfer catalyst(PTC). The yield of 3,4,5-trimethoxymandelic acid was 71.7%. (2) The reduction reaction: n(3,4,5-trimethoxymandelic acid): n(Trimethylchlorosilane): n(sodium iodide):n(zinc) =1:1.2:1.2:3, “one-pot” reaction at 80℃for 5h, the yield of target product was 91.0%. The overall yield was reached to 65.2%. The structures of the target compound were confirmed by IR and 1H-NMR.