Provided are a three-dimensional printing on-line monitoring method and system, relating to the technical field of three-dimensional printing, so as to solve the technical problem that the existing three-dimensional printing on-line monitoring systems are limited in imaging volume and cannot realize full-longitudinal-depth imaging. In the three-dimensional printing on-line monitoring method, full-longitudinal-depth imaging monitoring of the whole printing process is achieved by longitudinal-depth segmented scanning of a printing solidified layer and based on a longitudinal automatic splicing algorithm, and synchronous micro-tomography imaging on-line monitoring of the printing is achieved by guiding printing parameter optimization and control of a next depth-increased segment by using in real time a result feedback of the longitudinal-depth segmented being scanned; and at the same time of completing the manufacturing of the printed product, a three-dimensional high-resolution global image of an internal structure of the printed product is acquired, thereby completing quality control.