The invention discloses a cracking assistant for improving a low-carbon olefin concentration. The cracking assistant comprises 10 to 75wt% of a phosphor and transition metal-modified MFI molecular sieve, 0 to 60wt% of clay, and 15 to 60wt% of an inorganic oxide binder. There is not an absorption peak at the wavelength of 1633cm<-1> in a phosphor and transition metal-modified MFI molecular sieve infrared spectra obtained by trimethylpyridine as a probe molecule. Phosphorus distribution D is greater than or equal to 0 and less than or equal to 0.8. One or more transition metals adopted by the phosphor and transition metal-modified MFI molecular sieve are selected from Fe, Co, Ni, Cu, Mn, Zn, Sn and Bi. The cracking assistant is used for petroleum hydrocarbon catalytic cracking, can improve a catalytic cracking liquefied gas yield, can improve a propylene concentration and an isobutene concentration of liquefied gas, can reduce a dry gas yield and can improve a ratio of ethylene to dry gas.