Aiming at the weak output signal of photovoltaic detector in the laser flash thermal diffusivity measurement device, a weak signal amplification and conditioning circuit is designed. The input protection ring technology is used to enhance the anti-interference ability, and the electronic components are selected to improve the measurement accuracy. The Kelvin switch technology is used to realize the automatic switching function to eliminate the interference of the on-resistance. The experimental results show that the circuit has good repeatability, small fluctuation and long-term stability, which provides an important technical guarantee for the application of laser flash thermal diffusivity measurement device.