The magnesium slag is rich in Ca, Mg, Si elements, in which most of the arable land in China is deficient. Based on this, a new process for producing calcium-magnesium-sulfur-silicon(Ca-Mg-S-Si) fertilizer with magnesium slag is put forward by grinding and acid treatment. It is indicated from the trial production test that the optimum grinding diameter is about 60μm to producing the Ca-Mg-S-Si fertilizer, where the content of effective ingredient is more than 40%. Furthermore, pot experiments are carried out by choosing the maize and the radish as research objects. It is shown that the efficiency of self-made fertilizer is better than similar secondary-nutrients fertilizer on the market, which has the advantages of improving the lodging resistance ability, promoting the growing in later stage, and greatly reducing growing period of crops. Therefore, the application of the new technology for producing Ca-Mg-S-Si fertilizer by magnesium slag has tremendous economic and social benefits.