Two problems affect the quality of the small raindrop measurement: the signals of small raindrops at the receiver of optical disdrometer are very weak; it is difficult to detect signals from a very noisy background. Based on the digital lock in amplifier (DLIA) technique of weak signal detection principles, a digital lock in amplifier with OMAP3530’s DSP Core is constructed to solve the problems. The system is not only simple in structure, but also easy to implement. It can overcome zero drift and none linear in analog devices. Computer simulation results show that it can amplify weak signals constantly, inhibit the noise amplification of other frequency, and increase Signal to Noise Ratio (SNR). It can improve the detection of small drops in optical disdrometer effectively.