Current applications for photoelectric detection based on PSD were mostly with the incidence of laser, whose effective signal was highly stronger than that of background. A method of light-spot space position measurement under strong background based on PSD technology was proposed to enlarge the range of measurement. The pre-processing circuit was designed to convert the weak current signal from PSD to a voltage signal and finally to a digital signal from high-speed AD transformation. Influence on the results of PSD measurement form the background signal was analyzed. A method for extracting effective signal and accurate calculation for it was proposed by subtracting background and dark current signal. Related experiments with LED light-spot under different outdoor backgrounds show that this method can achieve 200 kHz high-speed measurement. Because of eliminating the influence from different background and dark current signal, linearity error of the measurement under strong background can reach the range of ±1.2%.