This paper presents the design and simulation of a lock-in amplifier(LIA) together with phase-shift function, applying in processing small signals buried in noises. A lock-in amplifier is equivalent to a narrow-band filter. It consists basically a phase sensitive detector followed by a low-pass filter. We designed the circuit based on the phase-sensitive detector AD630, together with a phase-shift circuit which can make the phase of reference signals coordinate with the phase of input signals. Simulation experiments verified the LIA can exstract useful signal from a noise signal approximately 1000 times in amplitude.