Firstly, research progress of complex dynamics of the Kopel system is described. Based on the bifurcation and normal form theory, a new proof of Neimark-Sacker (N-S) bifurcation of Kopel system is given, and approximate expression of the invariant curve generated by N-S bifurcation is obtained. Finally, the N-S bifurcation and invariant curve are numerically simulated, which verifies the theoretical results, and the mutual effect between the perturbation parameters in N-S bifurcation and the parameters in the approximate expression of invariant curve is analyzed. All analyses are supplement to the existing research of the Kopel system and provide a theoretical support for the intrinsic law of economic phenomenon.