The invention discloses a multi-pass laser amplification method and a gain module thereof, which belong to the field of laser design and relate to the design of a gain medium in a solid laser. A lath gain medium with four chamfered apex angles is utilized, laser is zigzag transmitted along an optical path between two large surfaces of a seed light lath, and total reflection is carried out among four sides of the lath for multiple times, thus increasing the transmission length of the laser path in the gain medium of smaller volume, extracting the energy more fully, offsetting the optical path difference caused by temperature gradient simultaneously and overcoming the defect of poor beam quality caused by the optical path difference to a certain degree.