Abstract: In order to convert weak optical signals into electrical signals effectively for post-processing, a weak optical signal detection circuit is designed. The circuit consists of a photoelectric conversion circuit and a preamplifier circuit. The photoelectric conversion circuit was implemented with a low input bias current operational amplifier AD549. The preamplifier circuit was implemented with a logarithmic ratio circuit using symmetric transistors, and was compared with the preamplifier composed of IC LOG100 under the same conditions. Measurement results show that the amplifier circuit can amplify weak input signals lower than 1nW and noise can also be suppressed efficiently, while the circuit composed of LOG100 has a relatively poor performance in noise suppression.