Steam distillation could improve the oil recovery effectively during thermal recovery process of heavy oil. However，steam distillation mechanism and its effect on oil properties was still not clear. Aiming at these problems，three groups of steam distillation experiments in different conditions were conducted with a 3- D physical model，including saturated steam，superheated 10 ℃ steam and superheated 40 ℃ steam. The results show that steam distillation ratio of crude oil is greatly affected by degree of superheat of injected steam. The higher the degree of superheat is，the higher the steam distillation ratio of crude oil is. Steam distillation ratio of crude oil in different conditions is 11.158%，10.903% and 10.423%，respectively. With the increasing injected volume，the distillation ratio increases. When the injected volume reaches a certain value，the distillation ratio will no longer increase. The experiment results of the density，viscosity，composition，carbon number distribution of crude oil and distillation fraction were compared to those before the experiment. The results show that light components of smaller carbon number were distilled off firstly by steam distillation，followed by heavier components. During steam distillation，the density and viscosity of residual oil increase，and the contents of saturated hydrocarbons and gum decrease，and the distribution region of carbon number shifts toward high carbon. The composition of the crude oil is changed.