BOG (Boiled off Gas) compressor plays an important role in the BOG treatment process. The suction temperature of BOG compressor is lower than -120 ℃, which results in great thermal stress and deformation during the start-up process. Therefore, it is important to carry out transient thermal analysis to ensure BOG compressor to start-up safely. Starting-up options used for BOG compressor include direct start-up and precooling start-up. This paper took 1st stage cylinder of BOG compressor as the research object. Heat transfer model of the cylinder was established. The temperature and stress fields for two different start-up methods were analyzed on the basis of finite element method. Compared with direct start-up process, the temperature gradient of cylinder is smaller for precooling start-up, and the cooling rate and stress growth rate of cylinder is gentler correspondingly. The difference of thermal stress and total deformation of cylinder with different materials (S304 and QTANi35) were compared. The deformation of QTANi35 cylinder is only 1.4 mm at working temperature, while the deformation of S304 cylinder reaches 4.6 mm. Therefore, when the cylinder material is S304, precooling start-up is needed. The influence of different precooling start-up schemes on cylinder temperature fields and stress fields were studied. The longer the precooling process lasts, the smaller the temperature gradient and stress growth rate are, and the scheme of precooling 180 minutes is more suitable.