Abstract: Aiming at the detection of metal surface defects, a set of metal surface defect detection system is designed. The system uses DDS (Direct digital frequency synthesis) algorithm to output sinusoidal signal with controllable amplitude and frequency to the excitation coil through AD 9767 chip. The high-frequency excitation signal in the excitation coil can excite the induction signal in the same frequency in the induction coil. The voltage signals of the excitation and induction coils are collected to FPGA(Field programmable gate array)through AD 9240 chip, and then the real and imaginary parts of the acquired induction voltage are decomposed by digital phase sensitive detection algorithm, and the effective complex impedance of the induction coil can hence be obtained. MAX 4656 chip and ADG 408 chip analog switch are used to control the switches of excitation channel and AD acquisition channel respectively to expand the number of channels. Experiments were performed to verify the feasibility of hardware and algorithm. Finally, the experimental results show that the system can generate sinusoidal excitation signal with controllable amplitude and frequency, and decompose the induced signal of the input channel to obtain the amplitude, phase, real part and imaginary part of the induced signal.