For the application problem of intake and exhaust plateau environment simulation device, the gas cavities from cylinder to crankcase were determined, and the numerical model of blowby gas flow through the gap was built according to the geometrical sealing structure of piston, piston ring and cylinder liner. Then the gas pressure and gas volume flow variation of each gas cavity were calculated by using the iterative method. The blowby influence and turbocharger shaft sealing were further analyzed under the altitudesimulated condition of the intake and exhaust pipes that was different from that of the crankcase vent. The discussion and verification were finally carried out. The results show that the blowby gas flow will increase by 0.4 L/min when the 4 500 m altitudesimulated condition of crankcase vent is conducted. Besides, the difference of gas pressures on both sides of compressor disk will cause a large negative pressure in the gap so that the lube oil for turbocharger bearing enters the intake pipe when crankcase vent is directly open to the atmosphere.