A control system of a desktop robot arm is designed. The control system used the open source circuit board Arduino Melzi as main board. The robot forward kinematics method and inverse kinematic method were transplanted to the open source Marlin firmware which was treated as the robots software. The purpose that using G-code to control the three degree of freedom (3-DOF) 3R robot was reached. Pronterface was used as upper computer in the control system which can control the robot online or offline. The test tesults prove that by using the control method of point to point transformation between right angle coordinate and rotational coordinate, the robot arm can have accurate trajectory, smooth motion, and good reliability.