Brassica napus, a crop that needs a high amount of magnesium (Mg), is mainly grown in the region where the soils are typically low in plant-available Mg. In recent years, Mg deficiency is becoming a potential factor limiting rapeseed yield due to the unbalance between large removal of Mg under high productivity cropping system and less input under current nutrient management that prioritizing the application of nitrogen, phosphorus and potassium. On the purpose to comprehensively and accurately evaluate the soil Mg status and the effects of Mg on seed yield, the present review summarized the main progress of Mg nutrition research in rapeseed plants. It is indicated that rapeseed requires 20-40 kg Mg/hm² for better growth and more than 50% of the soils in the main producing area lack sufficient available Mg. The application of Mg fertilizer greatly improved seed yield by more than 15%. Therefore, Mg is the fifth essential element in addition to nitrogen, phosphorus, potassium and boron, that needs to be supplemented by fertilization.