A catalyst used to prepare low-carbon olefin by conversion of petroleum hydrocarbon consists (by wt. percent) of clay 0-70, inorganic oxidate 5-90 and the zeolite 10-40, where the zeolite consists of the five-ring high silicon zeolite containing phosphorus and rare-earth; 25-75 and high silicon Y-type or Y-type containing rare-earth zeolite. Advantages: high stability for water-thermal activity, high yield of medium and low carbon olefin.