By studying the equivalent circuit of 1-demension position sensitive device(PSD),an expression of optical displacement which is independent of any specified systems is derived analytically in this paper.This expression shows that the ratio of the output resistance to the inner resistance of PSD and the asymmetry of two output resistors affect strongly both the linearity and the accuracy of measurement.The inner resistance of PSD changes significantly with the intensity of the light source.So the commonly used expression of optical displacement has to be modified when power light source such as laser beam is used.