Abstract: Since the reflectivity of two reflective surfaces of the corneal surface and the retinal pigment cortex is greatly different when measuring the axial length of eye, the intensity of the reflected optical signal belonges to the weak signal category under the premise of the incident light safety in the eye. In order to remove the noise and quickly extract the peak value of the signal for the calculation of the starting and ending point of the eye axial length, a weak optical signal detection system was designed. This system used PIN photodiode to receive weak optical signal and a weak light signal detection system was built coordinated with the I-V conversion circuit, automatic gain control circuit as well as the post-processing circuit. The experiment used ZIESS simulation eye to test. The results show that the designed detection system can detect the weak optical signal of about 0.77 nW at least, the maximum photoelectric conversion capacity can reach 4.5×10⁸ V/W, the signal-to-noise ratio is more than 9 dB, and the maximum error between the measured eye length and the actual value can reach 0.05%. This system is characterized by low noise, high gain and large dynamic range, which provides an effective solution for accurate measurement of the eye axial length.