The physical datum method that uses the laser beam as the physical datum is proposed.A high accuracy of the straight movement on the line above 0.01 μm/100 mm is achieved through applying the method of the closed-loop feedback control that uses the laser beam as the datum.The measuring accuracy above 0.05 μm is obtained by applying the self-making electric eddy displacement sensor to detect the geometric accuracy of the optical flat.It proves that the laser beam datum method can achieve the high accurcy of the straight movement and has the huge advantage.