In order to explore the influence of vertical smoke exhaustion shaft of urban shallow tunnel on smoke backlayering flow at upstream of the fire source, dimensional analysis method is used to establish the dimensionless expression of smoke backlayering flow at upstream of the fire source in Donghu Tunnel with vertical smoke exhaustion shaft; and numerical simulation method is used to study the influences of the heat release rate (HRR), longitudinal ventilation velocity, distance between shaft and fire source and the width and height of the shaft on upstream smoke backlayering flow. The results show that: (1) The upstream smoke backlayering flow length increases with the increase of HRR and the distance between the shaft and the fire source, and it becomes stable when reaches a certain value. (2) The upstream smoke backlayering flow length decreases with the increase of tunnel longitudinal ventilation velocity and shaft width; (3) The shaft height does not affect the upstream smoke backlayering flow length . (4) The HRR and the horizontal ventilation velocity affect the upstream smoke backlayering flow length more obviously than other factors. Moreover, a formula is innovatively deduced to predict the dimensionless smoke backlayering flow, which comprehensively considers the effects of the HRR, longitudinal ventilation velocity, distance between shaft and fire source and shaft width, and whose results are coincide with the numerical simulation results well.