Abstract: In order to on-line detect the diameter of the spot-welded nugget through the spectrum of the welding current,a liquid crystal displayer(LCD) system was developed to screen the welding current and its spectrum after fast fourier transformation(FFT).The digital signal processor (DSP) TMS320LF2407A and LCD TFTs6448b are indirectly interfaced to realize the data exchange with the same baud rate.Assembly language based program was embedded inside the C program to guarantee the real-time implementation of FFT.Experiments on spot weld of the mild steel with AC welding machine are pursued for the on-line status detection.The feasibility of the above displaying system in sample current collection,carrying out FFT,and displaying the waveform and its spectrum are validated.