The position error of four quadrant detector is analyzed.A new method for the center alignment of four quadrant detector and spot is put forward.The linear relationship between spot offset and the output voltage in a small area is obtained.The conclusions of the correctness is confirmed by the theoretical calculations

and by modifing relation curve

the relationship between the offset and the output voltage in the ideal conditions is described

a kind of method for improving the four-quadrant positioning system to eliminate the impact of the errors is provided

and the system of positioning accuracy is improved.