The preparation of dimethyldichlorosilance catalyzed by NaAlCl4 was calculated through B3LYP/LanL2dz and 6-311++G(3df,2pd) approaches. Conclusions are as follows: the rate-determining step of main reaction catalyzed by NaAlCl4/ Zn2+-ZSM-5 is the reaction of absorbing trimethylchlorosilane. The rate-determining step of main reaction catalyzed NaAlCl4/(AlCl2) +-ZSM-5 is the reaction that methyltrichlorosilane absorb intermediate(CH3AlCl2/ (AlCl2)+-ZSM-5) . The activation energies of the two reactions differ by 79.11kJ?mol-1. The introduction of (AlCl2)+ is good for reaction running and the Zn2+ is not. It matches well with frontier orbital analysis.