The invention discloses an on-line detecting method for laser welding T-type overlap joint gaps based on spectrum analysis. According to the on-line detecting method, aiming at the phenomenon that in the laser welding process of a T-type overlap joint, because the gaps between an upper plate and a lower plate are prone to being caused by welding deformation, test piece machining accuracy and others, the welding process is unstable, and defects are caused, the fluctuation frequencies of spectral line intensity of 500 nm-600 nm plasma spectrums along with time in the welding process are detected in real time, and the peak frequencies of plasma signal intensity under different gaps are calculated by using Origin software, so that the quantitative relation between the gaps and the peak frequencies is obtained, the gaps are detected, related parameters of welding are adjusted in time, and the gaps and the defects can be avoided. According to the on-line detecting method, a detecting system is simple in composition, high in interference rejection and good in engineering practicability, detecting light is high in signal intensity and accuracy, the laser welding T-type overlap joint gaps can be detected in real time, and the on-line detecting method is of great significance in actual production.