The invention discloses a method for identifying a deviation state of incident laser in an online manner. The method is used for carbon dioxide laser welding on metal plates which are of sandwich structures. The method includes shooting light-induced plasma cloud images by the aid of a high-speed camera in welding procedures; computing feature parameters of the plasma cloud images; computing process statistic quantities of the feature parameters of the images in set time periods; determining comprehensive indexes by the aid of a principle component analysis process; judging a position deviation degree of the incident laser according to the comprehensive indexes. The method has the advantages that the position deviation state of the incident laser can be accurately and quickly judged in the dioxide laser welding procedures for the metal plates which are of the sandwich structures, devices are simple, interference on the laser welding procedures can be prevented, and the method can be widely applied to the field of carbon dioxide laser welding on metal plates which are of sandwich structures.