Abstract: Mode (transverse mode) is one of the key parameters, which could be deduced from the status of the fiber lasers with different mode properties. The relationship between mode and beam quality is analyzed by numerical calculations, based on which it is pointed out that mode decomposition is the key to well understand the mode constitution and beam quality, then the common mode decomposition techniques are introduced. Aimed at mode instability (MI), which is a new phenomenon that prohibits power scaling of fiber laser, various parameters affecting the threshold of MI are summarized, then the physical mechanism and technique to increase the threshold are concluded. The recent progress on mode control of fiber laser is introduced from the viewpoint of high-order-mode suppression and structured light field generation.