Abstract: In order to obtain the interference signal in a high-precision heterodyne interferometer,a new design of gain-controlled weak-signal detector was presented.In the detector circuit,a low noise photoelectric conversion pre-amplifier was designed based on AD645 to convert the weak interferometric beam effectively.A gain-controlled main amplifier was designed to guarantee the dynamic range for the signal and applicable for various processing circuit.A biquad band-pass filter was introduced to minimize the noise and the temperature drift.It is verified experimentally that the output signal of detector has high signal-to-noise ratio,large dynamic range and high gain.The detector with high performance can be used in high precision interferometers.