There is provided an on-line detection system and method for a weld of a steel strip, which can emit a laser beam onto the surface of a steel strip moving at a high speed and measure the reflectivity of the laser beam reflecting from the same, thereby detecting the weld of the steel strip easily on-line. In the on-line detection system, reflectivity measuring means emits a laser beam onto a moving steel strip and continuously measuring the reflectivity of the laser beam returning from the surface of the steel strip, and signal processing means detects a weld of the steel strip based on change in the reflectivity measured on the weld.