Abstract: In order to improve the ventilation capacity of the auxiliary fan without wind wall, the calculation method of the optimal outlet radius of the ejector was obtained by the theoretical analysis of the auxiliary fan without wind wall, which is verified by numerical simulation. It has certain accuracy and rationality to judge the ventilation effect by the wind volume and the increase rate of the roadway. This paper introduced the calculation method of the optimal outlet section and summarized the influence of different outlet section on the auxiliary fan ventilation.