Abstract: In order to solve the problem of unqualiifed banded structure in microstructure of Q345B thick steel plate, the formation mechanism and influencing factors of banded structure were analyzed, and the process improvement measures were put forward. The results show that when the steel is cooled from the austenite phase zone, the carbon element in ferrite accumulates, until the temperature decreases to the austenitizing temperature A_r1, the retained austenite transforms into pearlite, and the pearlite accumulates in the rolling process, and finally forms ferrite and pearlite banded structure. The main factors affecting the formation of banded structure are heating temperature and finishing temperature. The banded structure of Q345B steel plate can be improved by increasing finishing temperature and adding water cooling treatment after rolling.