For the complexity of the problem of weak signal detection under the communication environment, lockin amplifier technique is used. The weak signal as input is for coherent reception with the phase reference signal that is adjustable, the noise mixed with an AC signal is transformed into a narrowband DC signal. System consists of a signal preprocessing circuit, a reference signal phaseshifting circuit, a phasesensitive detection circuit, the narrowband lowpass filter module. Wherein the reference signal phase shift circuitry provides the reference signal with the same frequency demodulation to improve the accuracy of phasesensitive detection circuit AC signal into a DC signal, the narrowband lowpass filter is used to remove broadband noise, accuracy is improved by many times for phasesensitive detection circuit measurement and least squares linear. This method ensure that the SNR is greater than 60 dB. when the signal magnitude within 10 μV-1 mV, detection error does not exceed 6% when noise is mixed with.