Based on the microscopic phase-field theory, the inversion formula of the connection between the first nearest neighbor interchange potential W1 and the long range order parameter η, i.e. was concluded according to the formula for the connection between occupation probability and free energy by Khachaturyan. According to the formula, W1 of L12 structure can be simply calculated under different conditions of atomic concentration and temperature only by substituting the temperature and atomic concentration of L1 structure transformation point. The calculated result is close to the numerical value by first-principle and other methods. At the same time, it is found that W1 of L12 structure was affected by atomic concentration and temperature in the phase diagram range of precipitated L12.