The invention discloses a controllable gain amplifier. The amplifier comprises an amplifying circuit and a gain control circuit, the amplifying circuit comprises an amplifier and a group of input resistance circuits which are respectively arranged at the positive input end and the negative input end of the amplifier; and the gain control circuit is connected with one group of input resistance circuits and is used for simultaneously controlling the on-off of multiple stages of input resistors in the input resistance circuits at the positive input end and the negative input end of the amplifierso as to control the amplification factor of the amplifier. Different from a gain control amplifier of a traditional R-2R network, the gain control amplifier has the advantages of being high in precision, easy to implement and the like, the problem that the resistance error in an amplifying circuit is large is solved, and the gain control amplifier has the high-precision controllable characteristic.