The invention relates to a hydraulic gap adjusting mechanism of a quenching and tempering device and a using method of the hydraulic gap adjusting mechanism. The hydraulic gap adjusting mechanism comprises a fixed frame, a lower roller connected to the fixed frame in a shaft mode and an upper roller located above the lower roller, the two ends of the upper roller are in shaft connected with a rotating bearing seat, the rotating bearing seat is arranged on a rotating shaft in a sleeving mode, a transverse plate and a gas adjusting plunger are further connected with the fixed frame fixedly, a gap adjusting cylinder for supporting the rotating bearing seat is fixedly arranged on the transverse plate, a pressure rod is further arranged on the transverse plate in a penetrating mode, the pressure rod is hinged to the rotating bearing seat, and a spring with one end jacked to the transverse plate is arranged on the pressure rod in a sleeving mode. According to the hydraulic gap adjusting mechanism, through the rotation of the upper roller and the lower roller, the straws and other crops are crushed, the rotating bearing seat rotates around the rotating shaft, the upper roller can be madeto move up and down, the fact that the pressure of the upper and lower rollers on the material during working is ensured under the function of the spring, the consistent width of a channel between thetwo rollers can be ensured through the hydraulic gap adjusting mechanism, meanwhile, hydraulic gap adjusting is convenient, and the same and stable pressure at both ends of the upper roller can be maintained.