The invention discloses an automatic soil pressure compensating device for adjusting quasi-rectangular shield soil pressure. The device comprises an outer ring barrel body, a propelling barrel body and a sealing structure. The outer ring barrel body is arranged on a soil bin shell of a quasi-rectangular shield. The propelling barrel body is arranged in the outer ring barrel body and connected with the outer ring barrel body through a telescopic oil cylinder. A propelling opening for the propelling barrel body to penetrate through is formed in the soil bin shell. The sealing structure is arranged on the inner wall of the outer ring barrel body and blocks the gap between the outer ring barrel body and the propelling barrel body. According to the automatic soil pressure compensating device composed of the outer ring barrel body and the propelling barrel body installed in the outer ring barrel body through the telescopic oil cylinder, the outer ring barrel body is installed on the soil bin shell of the quasi-rectangular shield, the propelling opening for the propelling barrel body to penetrate through and enter a soil bin is formed in the soil bin shell, the propelling barrel body is driven by the telescopic oil cylinder to move forwards and backwards to be pushed into or drawn out of the soil bin, the size of the soil bin is changed, and the effect on adjusting the soil pressure in the soil bin is achieved; meanwhile, by means of the sealing structure, the effect on preventing invasion of external mud can be achieved.