Abstract: Molecular distillation simulation of free fatty acid separation from rapeseed oil soapstock was carried out on the basis of the model and estimation of components thermodynamic properties. It provided basis for efficient preparation of biodiesel subsequently. Model values of degree of evaporation and free fatty acids content in residue were compared with experimental values. Results show that the tendency of model in process of simulation is similar to that of experimental value. Degree of evaporation increases with the increase of evaporator temperature. The relative error between model and experimental value is below 16%. The free fatty acid content in residue decreases when evaporator temperature increased. Experimental free acid content is higher than the model value, while the relative error decreased sharply with the increase of evaporator temperature. 50% of relative error is reduced when evaporator temperature of 453 K compared with evaporator temperature of 42