In allusion to the photoelectric signal detection or data acquisition under the condition of the weak photoelectric signal or weak signal to noise ratio, the modulation/demodulation circuit based on the autocorrelation detection principle is proposed. It is aimed at the photoelectric intensity adjustment of the modulation sensor and the detection of weak signal.The circuit uses pseudo random code and optical fibre sensor to modulate the weak signal and detects the weak signal according to the output light intensity. Meanwhile, the numerical simulation for the sensing circuit with strong noise is carried out. According to the simulation results, photoelectric signal detection and data acquisition based on the weak signal to noise ratio conditions are realized effectively.