Polycarbosilanes (PCSs) are synthesized at high temperature under high pressure. Synthesis processing and vacuum distillation of PCS are investigated. The results show that the molecular weight and distribution of PCS can be roughly controlled by synthesis conditions. With the increase of the reaction temperature and time, the molecular weight grows with a broadened distribution. High molecular weight PCS appears when the reaction temperature is higher than 450℃ with reaction time over 6h, or 460℃ with 4h, respectively. Super-high molecular weight PCS can be obtained if the reaction temperature and time still keep on increasing. The low molecular weight PCS content decreases while the vacuum distillation temperature increases. The molecular weight of PCS, therefore, increases with a narrow distribution. When the vacuum distillation temperature increases 50℃ from 250℃, the low molecular weight content in PCS decreases about 8% with a drop of polydisperity index about 0.3 and a grow of average molecular weight of high molecular weight about 1000.