Frequency conversion characteristics, produced by interact between laser and the object, are of great significance for target detection. The echo signal of frequency up-conversion has weak strength, pulse stretching, phase shift characteristics etc. According to this echo feature, we use the PIN photodiode as detector, and extract echo signal using lock-in amplification technology. With the help of the preamplifier circuit and AC signal amplifier circuit, the transformation and amplification of echo signal can be achieved, as well as phase sensitive detector, which is used to realize echo signal extraction. Echo detection system is set up and experimental tests show that detection system based on lock-in amplifier can suppress noise signal, as a consequence, the system can extract echo signal from background of big noise. When target at 1.5 meters in front of detection system, the output signal-noise ratio (SNR) of the system is 16.9 dB.