Abstract: Under the classical mechanics frame, this paper derive the expression of momentum theorem and deliberate the condition of momentum conservation in a general non-inertial frame, in translation and rotational motion. Then this paper formulize the cognition of momentum conservation on "close system" through surveying the motion in a system that hasn't any interaction with outside force in non-inertial system. As a result, it is pointed out clearly that since inertia force takes the role similar to outside force, therefore, closed system does not exist relative to non-inertia system.