Disclosed is a method and system for determining a motion path of a mechanical arm in which, after a kinematics model of a mechanical arm is established, multiple objective optimization functions are constructed according to the model, theoretical coordinates of a tail end of the arm, and an arm length. If a number of individuals in an evolutionary population, that makes both lateral and longitudinal error optimization functions in an optimization function monotonically increase or decrease, reaches the threshold, an optimal solution is determined using a single-objective evolutionary algorithm; otherwise, a multi-objective evolutionary algorithm is used. The multi- or single-objective evolutionary algorithms can be adaptively selected according to the number of individuals in the population that makes the lateral and longitudinal error optimization functions change. Therefore, short time consumption of single-objective and high accuracy of multi-objective are combined to quickly and accurately plan a motion path of the mechanical arm.