The near-infrared signal of the spaceborne solar spectrum monitor is usually weak and susceptible to external interference. To precisely detect the signal, a lock-in photodetection system based on InGaAs infrared detector was designed. The noise characteristics of detection system were analyzed, and it is found that suppressing the preamplifier noise can significantly reduce the overall noise. The PCB layout of preamplifier was optimized. Guard rings, a guard plane and a via fence were placed in order to strengthen the guard and shielding of input photocurrent signal. A tungsten lamp was used to simulate the solar spectral intensity in orbit and ground tests were conducted. The results show that the response linearity of the photodetection circuit to the incident light intensity is 99.95%. The signal-tonoise ratio (SNR) of each wavelength within the band is significantly improved compared with the previous solar spectrum monitor.