Evolution of chemical composition during phase decomposition and growth and the mechanisms of the directional interface migration during exterior load for Ni75Al7.5V17.5 alloy was investigated in this study, which is based on microscopic phase-field method. The results show that V which is needed for the growth of DO22 in the early stage came form the ordering domain among L12, V came from the interior of L12 in the later stage, and the speed for DO22 in the early stage was faster than it in the later stage. They formed five types of interfaces, and A-type was the main type for interface migration. It had a few A and B type interface in the early stage, and morphology changed little. With the rise of the number for A-type interface, the growth and decomposition for phases were accelerated, the number of A-type interface reduced and the number of B-type interface increased, and alloy reached balance fainlly. A-type interface migrated along stress during compress strees so that DO22 grew along this direction, and rafting structure of alloy formed.