The invention discloses a forage grass planting method based on mixed cropping of oat and common vetch. The method comprises the following steps of fine soil preparation before sowing and combined application of N fertilizer, P fertilizer, K fertilizer and organic fertilizer according to the soil fertility; variety selection, wherein the seed quality reaches the national level 3 or above, oat for the areas at the altitude of 3,000 m or above is selected from early maturing varieties such as Qingyin No.1, common vetch for the areas at the altitude of 3,000 m or above is selected from early maturing varieties such as Lanjian No.1, oat for the areas at the altitude of 3000 m or below is selected from the varieties such as Qinghai sweet oat, and common vetch for the areas at the altitude of 3000 m or below is selected from the varieties such as Ximu 324; determination of the sowing amount, wherein the sowing amount of the oat and common vetch is determined according to respective seedling conservation numbers, the seedling conservation number of the oat is 4,500,000-5,250,000 plants per hectare, and the seedling conservation number of the common vetch is 3,000,000-3,750,000 plants per hectare. The method has the advantages that the fertilizer application amount under different soil fertility levels is stipulated; the selection of planting varieties for mixed sowing forage grass land in different areas is stipulated; a method for planting in the mixed sowing forage grass land is stipulated; land resources can be comprehensively utilized, natural resources such as light, heat, fertilizer and water can be utilized intensively, the cultivation intensity and the planting cost can be reduced, fertilizer application is reduced, and the land resources can be protected.