The invention discloses a method for preparing low-carbon alkene by the catalytic thermal cracking of petroleum hydrocarbon, which belongs to the technical field of the catalytic thermal cracking. In order to solve the prior problems of solid particle agglomeration and complex preparation process caused by performing homogeneous phase treatment of a solid catalyst and then performing catalytic thermal cracking reaction, a slurry containing non-sodium type molecular sieve particles and the petroleum hydrocarbon to be cracked contact each other in a reactor to perform cracking reactions. Compared with the conventional thermal cracking process, the method improves the yield of ethylene, propylene and butadiene, obviously improves the ratio of the propylene to the ethylene, and reduces the yield of methane.