In order to study the reasonable range of flattening rate for shallow buried bias pressure small clear distance tunnels, based on the theory of surrounding rock deformation and numerical simulation, the stability of tunnel surrounding rock under different flattening rate conditions was analyzed. The value of flattening rate for small clear distance tunnels was discussed, and the rationality of the flattening rate range was verified by on-site monitoring data. Finally, the reasonable range of flattening rate for small clear distance tunnels was obtained. The results show that the flattening ratio of small clear distance tunnels has an impact on the distribution of plastic zone in surrounding rock, surface settlement, horizontal displacement of intermediate rock, arch settlement, and support shape variables. Based on the basic value of 0. 432 flattening ratio, as the tunnel flattening ratio continues to increase, the stability of the surrounding rock of the entire tunnel continues to decrease, with 0. 432 ~ 0.654 being the optimal flattening ratio range for the tunnel. The average error between the calculation results of the tunnel model and the on-site measured data is between 5% and 14%, which verifies the reliability of the numerical simulation and proves the credibility of the calculation method. Select the optimized tunnel section for construction and compare it with on-site monitoring data to verify the correctness of the optimized flatness ratio value.