Provided is a weak-light-signal multistage amplifying circuit. The circuit is characterized in that the circuit comprises a photoelectric detector, a prefixed amplifying circuit, a three-stage operation amplifying circuit and a one-stage amplifying detection circuit; the photoelectric detector is connected with the prefixed amplifying circuit, and the prefixed amplifying circuit is connected to the three-stage operation amplifying circuit and the one-stage amplifying detection circuit; after the photoelectric detector receives light signals, electric currents are generated, voltage signals are output after the electric currents go through the prefixed amplifying circuit, and the voltage signals are output to two operational amplifying circuits at the same time, wherein in one circuit, the voltage signals go through the one-stage amplifying detection circuit and output direct-current signals for detecting the direct-current magnitude of light signals, and in the other circuit, the voltage signals go through the three-stage operation amplifying circuit and output voltage pulse signals for detecting light-intensity changes. The weak-light-signal multistage amplifying circuit achieves the accurate measurement of micron-level matter, and can effectively suppress disturbance, improve the signal-to-noise ratio and obtain effective light signals.