The invention belongs to the technical field of the wireless digital communication, particularly relates to an automatic gain control system in an orthogonal frequency division multiplexing receiver and a method. An antenna of the orthogonal frequency division multiplexing receiver system is sequentially connected with a band-pass filter, a radio-frequency amplifier, an orthogonal demodulator, an analog-to-digital converter, a time domain power processor, a gain controller, a baseband digital gain amplifier, a frame synchronic judger, a frequency domain balancer, a decoder, a frequency domain processor and a gain generator. The time domain power processor, the gain controller, the baseband digital gain amplifier, the frequency domain processor and the gain generator form an automatic gain control system. The frequency domain processor consists of a comparison judger and an energy estimator. The automatic gain control system can prevent the valid signal from being submerged by slowly-decreasing signals, overcomes the problems such as the invalidity of synchronic correlator caused by the signal oscillation because of the variation of the gain, obtains a stable gain value, and improves the frame detection rate and the decoding success rate.