An automatic gain control（AGC） system is designed

which works in analog and digital mode. The analog part adopts digitally controlled amplifier and fixed attenuator

the digital part adopts the feedforward gain control structure based on average energy

and combines Newton iterative algorithm and coordinate rotation digital calculation method to simplify the realization of digital logic

making full use of the characteristics of flexible digital control and simple analog control. The simulation of the digital part in the simulation environment of MATLAB 2014a software shows that the final output signal of the system is stable

the steady-state response speed is increased by nearly 2 times and the signal impact phenomenon is significantly reduced

and the algorithm logic implementation is implemented on the field-programmable gate array（FPGA）

which further verifies the applicability of the system design.