The invention discloses a direct liquid-cooled matrix distributed gain laser based on polarized multi-pass side pump, related to the technical field of high-energy laser. Gain units (I1, I2) comprisedirect liquid-cooled matrix distributed gain modules (7, 17); a first pump module and a 1/2 wave plate are arranged on one side of the gain module, and a second pump module is arranged on the other side; the first pump module comprises a first diode laser matrix pump source, a first fast-axis shaping mirror, a first slow-axis shaping mirror, a first reflecting mirror and a first polarizing beam splitter successively; and the second pump module comprises a second diode laser matrix pump source, a second fast-axis shaping mirror, a second slow-axis shaping mirror, a second reflecting mirror, anda second polarizing beam splitter successively. Compared with the prior art, the invention has the advantages such as high output power, excellent thermal management mode, good output laser beam quality and the like, and has an important application prospect in the field of high-power lasers.