The invention discloses a preparation method of a catalyst used for cracking crude oil residual oil. The method comprises the following steps: stirring modified silica, alumino silica gel and water, adjusting the pH value of the above obtained solution to 1-1.6, stirring the solution, adjusting the pH value of the stirred solution to 6-6.8, curing the adjusted solution, adding a molecular sieve, stirring the cured solution and the molecular sieve, drying the obtained mixture, calcining the dried mixture, washing the calcined mixture, and drying the washed mixture to obtain the catalyst used for cracking crude oil residual oil. The modified bentonite is prepared through the following processes: stirring sodium carboxymethylcellulose and water, adding glucoamylase, carrying out ultrasonic treatment, carrying out vacuum distillation, adding bentonite and an ethanol solution, stirring all above substances, filtering the obtained mixture, washing the filtered mixture, carrying out vacuum drying, and grinding the dried mixture to obtain the modified bentonite.