The invention provides a device and a method for automatically adjusting classical gain in a continuous variable quantum entanglement source. The device comprises an optical parameter amplifier, a photoelectric detector, a central processing unit and a temperature controller, wherein a working temperature adjustable nonlinear crystal is placed in the optical parameter amplifier; the photoelectric detector converts an optical signal transmitted by the optical parameter amplifier to an electrical signal; the electrical signal is input to the central processing unit by an analog/digital converter; a signal output by the central processing unit is input to the temperature controller by a digital/analog converter; the temperature of the nonlinear crystal is controlled by the temperature controller; and the method for automatically adjusting the classical gain comprises the steps that the photoelectric detector monitors a transmission signal of the optical parameter amplifier, and the working temperature of the nonlinear crystal is adjusted by a detection result, so that a peak-peak value of the transmission signal is maximum, that is the classical gain is maximum. The device solves the problem that the existing continuous variable quantum entanglement source is difficult to operate, and lays a foundation for the device from a laboratory to a market.