A mechanism-parametric-calibration method for a robotic arm system is provided, including: controlling the robotic arm to perform a plurality of actions so that one end of the robotic arm moves toward corresponding predictive positioning-points; determining a predictive relative-displacement between each two of the predictive positioning-points; after each of the actions is performed, sensing three-dimensional positioning information of the end of the robotic arm; determining, according to the three-dimensional positioning information, a measured relative-displacement moved by the end of the robotic arm when each two of the actions are performed; deriving an equation corresponding to the robotic arm from the predictive relative-displacements and the measured relative-displacements; and utilizing a feasible algorithm to find the solution of the equation. When an ambient temperature changes or a stress variation on the robotic arm exceeds a predetermined range, re-obtaining the set of mechanism parametric deviations corresponds to a current robot configuration.