A process for preparing a catalyst for catalytically cracking hydrocarbon oils, which catalyst contains a substrate comprising alumina and molecular sieve(s), characterized in that, the pore distribution of said catalyst is 5-70% of the pore volume is in the <2 nm pores, 5-70% of the: pore volume is in the 2-4 nm pores, 0-10% of the pore volume is in the 4-6 nm pores, 20-80% of the pore volume is in the 6-20 nm pores, and 0-40% of the pore volume is in the 20-100 nm pores, based on the pore volume of pores having a size of no more than 100 nm, the pore volume of the catalyst is measured by the nitrogen adsorption volumetric method according to RIPP151-90, said catalyst contains 60-95 wt.% of a substrate, 5-40 wt.% of molecular sieve(s), and said molecular sieve(s) contains 25-100 wt.% of MFI-structured zeolite, 0-75 wt.% of Y-zeolite, and 0-20 wt.% of β-zeolite, which process comprises the steps of: mixing the substrate comprising alumina with the one or more molecular sieve(s), slurrying and spray-drying the mixture, characterized in that: a pore-extender is introduced in the mixing step and said pore-extender is one or more selected from boric acid and salts of alkali metals, the weight ratio of the pore-extender to the substrate being 0.1:100-15:100 based on the weight of the substrate, after introducing the pore-extender, a slurrying is performed for at least 5 minutes to dispense the pore-extender into the slurry and the slurry containing the pore-extender is aged in a static state under a temperature of 50°C to 80°C for 0.5 h to 3 h after the pore-extender is incorporated thereinto, and the salt of alkali metals is one or more of soluble salts of K, Na or lithium, and wherein the soluble salt of alkali metals is one or more of borates, phosphates, sulfates, nitrates, carbonates, and hydrochlorides. the pore distribution of said catalyst is 5-70% of the pore volume is in the <2 nm pores, 5-70% of the: pore volume is in the 2-4 nm pores, 0-10% of the pore volume is in the 4-6 nm pores, 20-80% of the pore volume is in the 6-20 nm pores, and 0-40% of the pore volume is in the 20-100 nm pores, based on the pore volume of pores having a size of no more than 100 nm, the pore volume of the catalyst is measured by the nitrogen adsorption volumetric method according to RIPP151-90, said catalyst contains 60-95 wt.% of a substrate, 5-40 wt.% of molecular sieve(s), and said molecular sieve(s) contains 25-100 wt.% of MFI-structured zeolite, 0-75 wt.% of Y-zeolite, and 0-20 wt.% of β-zeolite, which process comprises the steps of: mixing the substrate comprising alumina with the one or more molecular sieve(s), slurrying and spray-drying the mixture, mixing the substrate comprising alumina with the one or more molecular sieve(s), slurrying and spray-drying the mixture, characterized in that: a pore-extender is introduced in the mixing step and said pore-extender is one or more selected from boric acid and salts of alkali metals, the weight ratio of the pore-extender to the substrate being 0.1:100-15:100 based on the weight of the substrate, after introducing the pore-extender, a slurrying is performed for at least 5 minutes to dispense the pore-extender into the slurry and the slurry containing the pore-extender is aged in a static state under a temperature of 50°C to 80°C for 0.5 h to 3 h after the pore-extender is incorporated thereinto, and the salt of alkali metals is one or more of soluble salts of K, Na or lithium, and wherein the soluble salt of alkali metals is one or more of borates, phosphates, sulfates, nitrates, carbonates, and hydrochlorides. a pore-extender is introduced in the mixing step and said pore-extender is one or more selected from boric acid and salts of alkali metals, the weight ratio of the pore-extender to the substrate being 0.1:100-15:100 based on the weight of the substrate, after introducing the pore-extender, a slurrying is performed for at least 5 minutes to dispense the pore-extender into the slurry and the slurry containing the pore-extender is aged in a static state under a temperature of 50°C to 80°C for 0.5 h to 3 h after the pore-extender is incorporated thereinto, and the salt of alkali metals is one or more of soluble salts of K, Na or lithium, and wherein the soluble salt of alkali metals is one or more of borates, phosphates, sulfates, nitrates, carbonates, and hydrochlorides.