The invention discloses a catalytic cracking assistant for improving gasoline octane number barrel yield and a preparation method thereof. Based on a dry base weight, the assistant comprises 10-75wt%of an MFI structural molecular sieve containing phosphorus and loaded metal in terms of the dry base weight, 3-40wt% of a phosphorus aluminum inorganic binder in terms of the dry base weight, 1-30wt%of other inorganic binder in terms of oxide, 0-60wt% of second clay in terms of the dry base weight, and 0.5-15wt% of a metal additive selected from at least one of VIII group metals and manganese, zinc, gallium. The assistant provided by the invention can be applied to a catalytic cracking process to increase the content of isohydrocarbon in gasoline so as to increase the gasoline octane number,and at the same time can maintain the gasoline yield basically unchanged, thus improving the gasoline octane number barrel yield.