when screw vacuum pump works,the rotor will be deformed under the load of thermal and mechanical loads.If the clearance between rotors and the pump cavity is too small to compensate the deformation,the pump will be jammed.On the contrary,If the clearance between rotors and the pump cavity is too large the performance of the pump will be decreased because of the larger leakage.This paper studied the deformation of the rotors caused by the force load and thermal load to determine the obligated clearance between rotors and case.The 3D solid modeling of single thread and fxed pitch screw vacuum pump of 180L/s is established.Considering the convective heat transfer coeffcient changes along with the parameters such as pressure,the rotor will be divided into four section.the fnite element simulation with the calculated equivalent temperature and the convective heat transfer coeffcient applied on the corresponding position.Analyze the temperature of the solid rotor and the rotor whose axis runs water.The results showed that the cooling effect of the rotor whose axis runs water is obvious.Analyze the heat load of the solid rotor and the rotor whose axis runs water.The results showed that the structure of running water greatly reduces the thermal expansion of the rotor.Analyze the deformation of loads both heat and mechanical.The results showed that there is no difference between the thermal deformation and thermal coupling with only one percent disparity.Therefore,deformation of the rotor is mainly determined by heat loads.The results of this paper provides a theoretical basis for defning a reasonable clearance and structure of cooling.