basis and deionized water were added to prepare a slurry having a solid content of 370 g l to which 200 g ammonium sulfate was added the system ph was adjusted to 3 6 the temperature was raised to 90 u00b0 c and an exchange reaction was carried out for 1 2 h followed by filtration and washing the filter cake was calcinated under 20 water vapor at 600 u00b0 c for 0 5 h to produce a u201c two exchange two calcination u201d rare earth ultra stable y molecular sieve to a reaction kettle equipped with a heating mantle 500 g of the u201c two exchange two calcination u201d rare earth ultra stable y molecular sieve dry basis and a certain amount of deionized water were added to prepare a slurry having a solid content of 120 g l to which 42 g magnesium chloride hexahydrate was added the temperature was raised to 90 u00b0 c and an exchange reaction was carried out for 0 8 h followed by filtration washing and spray drying so as to produce the active component of a magnesium modified rare earth ultra stable y molecular sieve according to the present invention designated as modified molecular sieve.