The invention discloses a crystalline grain control method for a GH690 alloy shear cover of an explosion valve and belongs to the technical field of material machining. The method aims to solve the problems that the structure of a forge piece of the GH690 alloy material shear cover of the explosion valve is not even, and crystalline grains abnormally grow up. A heat treatment machining is organically combined with a forging machining, and the machining method that large deformation free forging is implemented at a high temperature (1020 DEG C to 1150 DEG C), small deformation foetal die forging is implemented at a medium temperature (710 DEG C to 850 DEG C), and the inhibition function of carbide second-phase particles on moving of a grain boundary is used for restraining the abnormal growth of the crystalline grains in forging machining of the shear cover in an auxiliary mode is provided. According to the crystalline grain control method for the GH690 alloy shear cover of the explosion valve, the forging piece forming and crystalline grain refinement are conducted synchronously, and the shear cover forge piece is obtained through heating twice; the machining limitation that a forging piece with the simple structure is subjected to high-temperature multiple-time hierarchical forging machining at present is overcome; the forging heating number is reduced; the follow-up heating treatment of the forging piece of the GH690 nickel-based alloy sheer cover is avoided. The structure with even carbides and refined crystalline grains of the GH690 alloy shear cover of the explosion valve forged through the method can be obtained.