The invention relates to a preparation method of a transition metal modified silicon-magnesium zeolite catalyst. The preparation method comprises steps as follows: a, magnesium sulfate is added to deionized water, concentrated sulfuric acid is added, and a solution A is obtained; b, a silicon source is added to the deionized water, and a solution B is obtained; c, the solution A is slowly added to the solution B, meanwhile, a template agent is added, and a solution C is obtained; d, the solution C is stirred for 0.5 h at high speed, and obtained gel is put in a hydrothermal crystallization kettle for crystallization; e, a solution obtained after crystallization in the step d is subjected to suction filtration, a mother solution is removed, a product is dried in a drying oven at the temperature of 100 DEG C for 24 h, and raw silicon-magnesium zeolite powder is obtained; f, the raw silicon-magnesium zeolite powder is added to a metallic nitrate solution and stirred for 2 h, a co-precipitation agent is dropwise added, pH is regulated to 10, a mixture is stirred and dried in the shade, and a catalyst precursor is obtained; g, the catalyst precursor is roasted at the temperature of 500-600 DEG C. The silicon-magnesium zeolite catalyst prepared with the method has the advantages of high catalytic activity and good stability.