The invention discloses a multistage amplifier which comprises a differential input stage, at least one intermediate amplification stage and an output stage. The differential input stage, the intermediate amplification stages and the output stage are sequentially in cascade connection with one another; the differential input stage comprises a first MOS (metal oxide semiconductor) transistor, a second MOS transistors, a third MOS transistor, a fourth MOS transistor, a first current mirror circuit, a second current mirror circuit, a first bias circuit and a first load circuit, source electrodes and grid electrodes of the first MOS transistor and the second MOS transistor are coupled with one another, and first voltages can be received by the grid electrodes of the first MOS transistor and the second MOS transistor; source electrodes and grid electrodes of the third MOS transistor and the fourth MOS transistor are coupled with one another, second voltages can be received by the grid electrodes of the third MOS transistor and the fourth MOS transistor, the source electrodes of the third MOS transistor and the second MOS transistor are coupled with each other, and first bias currents can be received by the source electrodes of the third MOS transistor and the second MOS transistor; a common grid electrode of the first current mirror circuit is coupled with a drain electrode of the third MOS transistor, and a first drain electrode of the first current mirror circuit is coupled with a drain electrode of the first MOS transistor; a common grid electrode of the second current mirror circuit is coupled with a drain electrode of the second MOS transistor, and a first drain electrode of the second current mirror circuit is coupled with a drain electrode of the fourth MOS transistor; the first bias circuit is suitable for providing bias circuits for the first current mirror circuit and the second current mirror circuit; the first load circuit is suitable for providing load for the differential input stage. According to the scheme, the multistage amplifier has the advantages of high gain and wide unity gain bandwidth.