Accurate detection of reference beam was one of the key factors to enhance the ranging accuracy in the laser active imaging system with direct ranging. The circuit design of detection of reference beam based on PIN detector was researched in detail. Firstly, the comparative analysis of performance and difficulty of power supply circuit between avalanche photodiode(APD) and PIN photodiode was conducted. Considering the power stability and designing difficulty of the reference beam, the PIN detector was selected to detect the reference beam in ladar. Two modes including normal power and current monitor existed in LT3482, and the difference when used to drive PIN detector was analyzed deeply. The transimpedance amplifier OPA657N with high gain-bandwidth product was used to convert the current of PIN detector to voltage, and the selection method of key external components was introduced. Also, points for attention in usage of MAX9601 were given. At last, the circuit was designed for experimental verification. The results show that voltage converted from the current of PIN detector through transimpedance amplifier is -3.7 V when the pulse width of laser is 10 ns and bias voltage applied to the PIN detector is 89.449 V. The pulse width of voltage is extended to be 15 ns, but the starting signal is ensured to be detected. The timing discrimination signal according to the reference beam could be given effectively by the module. PIN detector based on integrated high-voltage chip combined with constant threshold timing discrimination was feasible in direct ranging laser active imaging system detection of reference beam.