Abstract: This paper introduces the principle of real-time automatic gain control based on feed forwarding technology, and the realization of the critical technologies including digital compensation and synchronization, as well as, the specific application of communication analyzing instrument. The real-time automatic gain control makes full use of the delay feature of signal channel, and controls the synchronization of ADC precisely, and adjusts the signal gain in real-time, and compensates the error in digital domain, at last, this kind of technology expands the dynamic range of system ADC. This technology has been applied to the communication analyzing instrument; meanwhile, it can be extended to the field of radar, anti-electronic and receiver.