The invention provides a low-frequency balanced zero beat photodetector, which can be used for detecting quantum noises of a laser at the frequency of 50Hz-200kHz and measuring the compression noise spectrum of a non-classical light field in this band. The photodetector comprises a first PIN photodiode (1) and a second PIN photodiode (2) which are of high quantum efficiency, an AC coupling capacitor (3), a low-frequency transimpedance amplifying circuit (4), a DC sampling resistor (5) and a DC same direction proportion amplifying circuit (6). The AC output of the photodetector has the advantages of high gain, low noise, high common mode rejection ratio and so forth, and the photodetector also has a DC output and can monitor the change in the light intensity.