Abstract: The modular liquefied gas micro-propulsion system of Laval nozzle was simulated by using FLUENT software,and got each size of nozzle thrust and effective specific impulse,and gave the optimized results of nozzle.The nozzle has five sizes:inlet diameter,throat diameter,outlet diameter,contraction length and expansion length.Among them,the greater the throat diameter,the larger the thrust,the smaller the effective specific impulse; the greater the outlet diameter,the larger the thrust and effective specific impulse; when the contraction length is small,it affects the effective specific impulse,and almost has no effects on thrust; the effects of inlet diameter and expansion length are very small to be ignored.