A detection method in presented for mid-infrared laser signals with 4 kHz repeat frequency, 20 ns pulse width and 1 W average power. The long distance between the detector to the source resuts in weak energy output from the detector 30～60 μV, and the loop gain of the single stage of the amplifier can not exceed 100 times. Otherwise, overage gain will brings poor frequency characteristic, instability and high noises. So three-stage amplifier is designed, the total loop gain is 1000～15000 times, furthermore, an offset adjustment placed on the noninverting input to avoid the distortion of signal source, and high-pass filter is designed to reduce the noise of under 100 kHz and depress and attenuate the disturbing signals caused by high gain. Lastly, the effective and unusual method of data acquisition is put forward aiming at the characteristics of the mid-infrared short pulses.