A robotic arm system includes a robotic arm, a controller, and a parallelism calibration device. The robotic arm has a terminal shaft provided with an end surface. The parallelism calibration device is mounted to the terminal shaft and has at least one distance measuring device. The parallelism calibration method of the robotic arm system includes: first moving the terminal shaft to a calibration position near a reference plane; using the parallelism calibration device to measure the distance differences between at least three points on the reference plane and the end surface and transmit the measurement signal to the controller; and the controller according to the measurement signal to adjust the posture of the robotic arm for making the distances of the three points to the end surface equal; accordingly, this invention enables the robotic arm to achieve precise parallelism with respect to the working plane rapidly in any working situation.