Based on Fe—Mg distribution coefficient between olivine and melt, the MgO—FeO evolution diagram is built. The compositions of primary magmas can be estimated by the intersection point of straight line and curve which illustrates liquids in equilibrium with olivine. It is suitable for the system of olivine fractional crystallization, and its advantage is that it can be easily and visually used. It is also pointed out how the diagram is applied. To show how to recover the composition of primitive magma and to explain the petrogenesis, the samples from the Lijiang area of Emeishan large igneous province are taken for examples. Results show that composition for the MgO and FeO contents of Shiman primitive magma are 23.5% and 12.8% respectively, with larger melting fraction. Samples SM 14 and SM 15 are resulted from low olivine fractional crystallization, so their contents could approximately represent primitive magma of Shiman area. Daju primitive magma may be divided two types. One has 19.8% MgO and 11.3% FeO contents, with lower melting fraction. The other one has 23% MgO and 13.3% FeO contents similar to the Shiman, with larger melting fraction. Samples from the Daju all experienced large olivine fractional crystallization. Most basalts from the Emeishan large igneous province have MgO contents less than 8%, and include many clinopyroxene and plagioclase phenocrysts,, suggesting that these basalts can not be directly resulted from olivine fractional crystallization of Emeishan primitive magma.