The catalytic conversion method for preparing lower olefin and high-octane gasoline features that the catalytic conversion reaction of preheated petrohydrocarbon contacted with phosphorated and rare-earth-contg. pentacyclic high-Si zeolite catalyst is conducted under the conditions of 480-680 deg.C, 0.12-0.4 MPa for 0.1-6 seconds, the weight ratio of catalyst freed oil=(4-20)/(1), and the weight ratio of steam feed oil=(0.01-0.5)/(1). The resultant is separated into lower olefin and liquid. The catalyst may be reused by regeneration.