Mixed grain micstructures of GH4720Li superalloy with different coarse and fine grain ratios were prepared by forging deformation. The high-temperature tensile properties of different mixed grain micstructures at 650oC were tested. The quantitative relationship between mixed grain micstructures and high-temperature tensile strength was established, and the mechanism of influence of mixed grain structures on high-temperature tensile properties was revealed. The results show that the primary γ" phase distribution directly affects the evolution of the mixed grain structures during the high-temperature forging process of coarse crystal GH4720Li alloy. The more uneven the primary γ" phase distribution is, the easier it is to form the mixed grain micstructure after the deformation of the coarse grain structures. During the 650oC tensile process, the coarse grain size and volume fraction in the mixed grain micstructures significantly affect the tensile properties. With the increase of the coarse grain size and volume fraction, the high-temperature tensile strength of the alloy decreases slowly and then rapidly, while the plasticity decreases rapidly. The tensile strength shows a Hall-Petch quantization relationship with the equivalent grain size. But the effect of coarse grain size on the equivalent grain size is higher than that of fine grain size. The RD//<111> oriented grains are formed in the microstructure during high-temperature tensile deformation. The smaller the difference in the ratio of coarse and fine grains in the mixed grain micstructure, the fewer RD//<111> oriented grains are formed, the greater the dislocation slip obstruction, and the worse the high-temperature tensile property of the mixed grain micstructure.