The interaction between solitons of the nonlinear Schr?dinger equation is derived by the least action principle approach as a function of the soliton' s separation △ and their initial relative phase δ. The equation shows that two solitons with δπ/2 repel, and there is almost no interaction when δ=π/2, and that the interaction decreases exponential-ly as △ raises. The results obtained is well consistent with that of the experiment quantitatively.