Abstract: The catalytic cracking reaction of FCC naphtha was studied on La or Ce modified ZSM-5 zeolite under 550℃, atmospheric pressure and water vapor conditions. Experimental results suggest that the modification of rare earth La3+ and Ce4+ can increase the total acid amount of ZSM-5 molecular sieve, especially strong acid acidity, so FCC gasoline conversion rate and in particular the olefin cracking reactions obtain marked improvement. In addition, after the modification, the selectivity of the olefin cracking reaction and the yield of ethylene, propylene, butylene, especially propylene are also significantly increased. Molecular simulation results show that La3+ and Ce4+ locate at the corner in bent Z type channel of ZSM - 5 molecular sieve, 3~4? away from the hole center, making the volume of the bend decrease obviously. This leads to the increase of energy barrier for all reactions including cracking, cyclization, aromatization and polymerization, but the cracking barrier increases minimum which improves the selectivity of cracking reaction of olefins.