Based on the microscopic phase-field kinetic model and the microelasticity theory, the precipitation process and the atomic site occupation of γ′ phase of Ni75Al15Mn10 alloy were simulated. The results indicate that when the alloy is aged at 1273 K, L10 phase is formed firstly, and then gradually transformed into L12 phase with the development of the long range order parameter at the initial precipitation stage. Atomic ordering precedes compositional clustering, and γ′ phase precipitation is of the congruent ordering + spinodal mechanism. The volume of γ′ ordered phase is less than that of γ phase, and the volume ratio of the ordered to the disordered is about 60%. The β site is mostly occupied by Al atoms (the corner site of γ′ phase), while the αⅡ and αⅠ sites are mostly occupied by Mn atoms; moreover, the occupation probability at αⅡ site is higher than that of αⅠ site, Mn atoms mostly occupies the site of Ni atoms, and the precipitated γ′ phase is a kind of (Ni, Mn)3Al phase.