A robot arm control device includes a pressure sensing module, a workspace defining module and a control module. The pressure sensing module, arranged on a robot arm, detects whether an object hits or touches the robot arm to switch the operating mode of the robot arm. The workspace defining module includes a sensing region arranged on a peripheral area around the robot arm. The workspace defining module determines whether the object enters an operating space according to the position of the object in the sensing region, and sets the working range and the working mode of the robot arm according to which operating space the object has entered. The control module, connected to the robot arm, the pressure sensing module and the workspace defining module, switches the operating mode and outputs a motor driving signal to the robot arm according to the working mode of the robot arm.