The present invention provides an online laser leveling detection method of a 3D printer. The method comprises: (1), arranging a bar-shaped triangular reflecting prism and a bar-shaped photoelectric receiver; (2), fixedly mounting a laser emitter and a plane mirror on the side face of a printer head; (3), with the aid of a detection laser beam, fine tuning the mounting azimuths of an X-axis guide rail, a Y-axis guide rail, and the printer head till the motion levelness of the printer head is adjusted to meet requirements of the design accuracy of the printer; (4), with the aid of the detection laser beam, fine tuning the mounting azimuth of a printing platform till the mounting levelness of the printing platform is adjusted to meet the requirements of the design accuracy of the printer; (5), starting the 3D printing program; (6), in the printing process, detecting in real time whether a nozzle of the printer head generates faults including material blockage, shortage, and breakage. The present invention communicates detection signals of a laser detection system with a printer control system and designs the signal processing program to achieve detection of the motion levelness and the mounting levelness as well as monitoring of extrusion faults of the printer head.