The invention discloses a 2R underactuation plane mechanical arm control method. According to the method, a coupling relationship between a driving arm and an underactuation arm is analyzed through dynamics, an underactuation system dynamics equation is built, and a control target is completed in two stages; when the underactuation plane mechanical arm is located in a swing starting zone, a performance index function is designed according to the shortest time principle, then a Hamilton functional equation is designed according to the performance index function, the Hamilton functional equationand a system dynamics equation are simultaneous, and a computer is used for solving a swing starting zone control law; and in a linearization zone, a nonlinear system approximates to the linearization system, the linearization zone control law is designed according to a linear quadratic type stabilization adjustor principle, and the underactuation plane mechanical arm can be stabilized to a balance point in the vertical direction. According to the 2R underactuation plane mechanical arm control method, it can be guaranteed that on the premise that the 2R underactuation plane mechanical arm canbe stabilized on the balance point in the vertical direction, the control time of the 2R underactuation plane mechanical arm is greatly shortened.