In view of the particularity of the limited space structure of the step construction tunnel, the fluid mechanics analysis software FLUENT was used to determine the location of the press-fit ventilation duct, the distance from the air outlet of the duct to the working surface, the length and height of the step ,and then numerically simulated the influence of the flow field, analyzed the flow field under the influence of various factors and optimized the corresponding parameters. The results show that: 1) the stepped construction tunnel press-in ventilation is a limited attached jet in a limited space; 2) the air duct is located on the side to facilitate the timely circulation of air in the limited construction space; 3) when the step height reaches the air duct and When the air outlet is at a certain distance, the presence of the steps on the tunnel has a certain lifting effect on the jet; 4) The smaller the distance between the air outlet and the working surface of the upper step, the more likely to be offset but the smaller the vortex impact area. When the distance between the air outlet and the upper working surface is larger, the large eddy current is more likely to appear in front of the work on the step. The value of L1 is preferably 5-10m; 5) The shorter the step, the greater the effective range of the jet. When the length of the step exceeds a certain length, the impact on its flow field is small. The length of the upper step L2 is preferably 5 ~ 10m, but it should be less than L1; 6) In order to avoid space constraints and ensure the full development of the jet, the height L3 of the upper step should be less than the height of the air duct arrangement, should be 4-5m.