The atomic and molecular reaction dynamics for Si（1Dg）+H2（0，0）and H（2Sg）+SiH（0,0） have been studied on the potential energy function SiH2（X1A1） by Monte-Carlo quasi-classical trajectory approach.It is shown that the reaction Si（1Dg）+H2（0，0）has no threshold energy，and the principal product of this reaction is SiH2（X1A1）.However, the reaction H（2Sg）+SiH （0,0） gives a great number of products of the interchange reaction of H（2Sg）+SiH （0,0）→Si（1Dg）+H2（0，0），and it has also no energy threshold.