An online laser-welding monitoring system is performed to monitor the gap of overlap joint in fiber laser welding steel sheet. The results indicated that the gap of overlap joint have a huge impact on welding appearance but nothing to do with welding penetration. Amplitude of optical signal in time domain and frequency distribution in time-frequency domain has a significant change when the defect occurs in the laser welding process. Optical signals are analyzed using signal processing methodologies to reveal intrinsic interaction between optical signals and the gap, it also provide theoretical basis and experimental guidance for analyzing the dynamic behavior of plasma and pool.