The study gas multistage separation at the preliminary preparation of crude oil by means of computer simulation was performed. Different models have been used as the phase state equations in HYSYS software: Peng-Robinson, Grayson-Street-Choa-Seeder, Peng-Robinson-Twu, Suave-Redlich-Kwong, Twu-Sim-Tassone. The carry-over saving of liquid oil components into off gases approximately is 40–60% while optimizing thermobaric conditions of the separation was obtained. The semi-empirical model obtained on the basis statistical processing of the results of laboratory experiments was proposed. The proposed semi-empirical model has a slight deviation for the first and the second separators. There is a negative effect in the third separator, but it is not significant in comparison with the positive effect in the first and the second separators.