Synthesis method of spinning-grade polycarbosilane with high softening point

The invention discloses a synthesis method of spinning-grade polycarbosilane with a high softening point. The synthesis method comprises the following steps: adding polycarbosilane with a certain molecular weight and a low softening point and organosilane or silazane compound with a double bond -C=C- group; adding a solvent which is dimethylbenzene and evenly mixing; then adding a Si-H addition reaction catalyst; vacuumizing for replacing high-purity nitrogen; then heating to 80-90 DEG C and reacting for 1-2 hours firstly under normal-pressure or high-pressure conditions in the presence of the high-purity nitrogen, then heating to 300-350 DEG C at a heating rate of 45-55 DEG C/h, performing Si-H addition reaction, and preserving heat for reacting for 2-6 hours; distilling the solvent in the reaction or after the reaction to obtain a product. The synthesis method is simple, low in preparation cost and easy to produce in bath. Polycarbosilane fiber prepared by using the polycarbosilane with the high softening point as raw material through melt spinning is capable of realizing infusible treatment in a non-oxygen reactive atmosphere and beneficial to production of high-performance continuous silicon carbide fiber products.