A balanced multistage low-noise amplifier is proposed to address the problem that the acquisition of backscattered light mode signals in the backscattered light measurement of laser gyro resonance cavities requires low-noise amplifiers in specific frequency bands. The low-noise amplifier adopts a balanced multistage cascade structure, designs a special orthogonal coupler to achieve conjugate matching, adjusts stability based on negative feedback, optimizes noise for the bias circuit, selects impedance points based on noise and gain, and analyzes and innovates the structure of the orthogonal coupler from the perspective of power flow transmission. The design goal of " impedance matching and noise minimization" is finally achieved. Verification shows that the gain is up to 20dB at 2.3 GHz~2.7 GHz, the gain flatness is about 3dB, the overall input VSWR is less than 1.50, and the output noise of the photodetector is obviously suppressed. The design results can be referenced and applied to many scenarios such as signal amplification of high-frequency optoelectronic devices, mixer design, and TR components to solve the problem of low gain and high noise of single-branch low-noise amplifiers.