To reveal the distribution characteristics of ceiling radiant heat flux in full-scale tunnel fires，the numerical simulation research was carried out. Longitudinal ventilation velocities，heat release rates and effective fire heights were changed. The distribution of ceiling radiant heat flux in road tunnel vehicle fires，the maximum ceiling radiant heat flux and its position were analyzed. The influences of the longitudinal wind speed，the heat release rate，and the effective fire height on each parameter and their important degrees were revealed. Results show that under the natural ventilation，affected by the heat release rate and effective fire height，the radiant heat fluxes under the ceiling above the fire source surface show a uniform distribution，or the law that the middle radiant heat fluxes are low while the two sides are high. Under the longitudinal ventilation，the ceiling radiant heat flux increases and then decreases. The peak value of the heat flux occurs near the center of fire source and the downstream of fire source. At large effective fire source heights，the peak values of the maximum ceiling radiant heat flux are obtained under the condition of wind speed lower than the critical velocity. While reducing the effective fire height and increasing the heat release rate，The peak value of maximum ceiling radiant heat flux individually occurs both when the wind speed is less than and when it is greater than the critical wind speed. When the wind speed below the critical velocity，the maximum ceiling radiant heat flux presents right above the surface of the fire source，and it is weakly affected by the heat release rate，longitudinal ventilation and the effective fire height. When the wind speed above the critical velocity，the maximum ceiling radiant heat flux increases with the heat release rate，and it is less affected by the wind velocity. The research results can provide some reference basis and data supports for tunnel fire prevention and control and tunnel safety design.