The photoelectric detection principle four quadrant detector was introduced in this paper . The background light influences on four-quadrant detector were analyzed in detail in theory. The analysis results indicate that position error is linear with measurement position and nearly inverse proportion to the SNR. The prediction of theory is consistent with experi-mental results. Some methods of decreasing background light influences on QD were given in the end. The work makes it possible to evaluate such additional position error quantitatively and guide the application of QD.