Abstract: The present disclosure discloses a method of preparing tris-(2-chloroethyl)phosphite, comprising: continuously pumping phosphorus trichloride and ethylene oxide into a tubular pre-mixer for pre-mixing, respectively, and starting a refrigerant cooling unit of the microchannel reaction device simultaneously, to obtain a product tris-(2-chloroethyl)phosphite after complete reaction. The method according to the present disclosure not only significantly improves the content of the esterification product, thus increasing the production efficiency by 10 to 100 times than that of an ordinary tank reactor, but also reduces the residence time of the esterification product in the reactor from 110 hours to several minutes.