The methods of precipitation and blending to adjust the molecular weight and distribution of polycarbosilane (PCS) synthesized at high temperature under high pressure were investigated in the paper. The results show that, the high molecular weight PCS can be precipitated by adding certain amount of precipitator (such as acetone) into the PCS solution of favorable solvent (such as xylene). The higher the molecular weight of PCS, the earlier it precipitates when acetone is added to a degree above the critical value. The softening point, molecular weight and polydispersity index of PCS are therefore decreased. The molecular weight distribution of PCS is thus controlled. Blending of PCSs with different molecular weights and distributions is another method to adjust the molecular weight and distribution. The results show that, the softening point, weight average molecular weight and content of high, middle and low molecular weight of the obtained PCS are accorded with the rule of mixture (ROM) when two different molecular weight PCSs are blended together. Therefore, PCS with required softening point, molecular weight and distribution can be designed in advance according to ROM.