The invention provides a mixed molecular sieve catalyst, comprising 5-30 wt % Y molecular sieve, 5-30 wt % ZSM-5 molecular sieve and the balance of matrix component and binder component. The invention further provides a method of preparing the mixed molecular sieve catalyst, comprising the following steps: mixing the Y molecular sieve, the ZSM-5 molecular sieve with a matrix material, a binder precursor, a pore former, a dispersant and water to formulate an aqueous slurry; spray drying the slurry; and baking. The invention further provides a method of preparing olefins from methanol with the use of the catalyst. By use of the catalyst of the invention, the selectivity to C3 and C4 olefins is improved significantly.