Based on fast cook-off test results of free-loading solid rocket, numerical analysis of radial heat transfer in first stage of the motor's response to fast cook-off is carried out. The results show that motor's temperature distribution is affected by the external high temperature flow field, which has a non-uniform distribution pattern with more heat on both sides, less heat on the bottom, and least heat on the top. Until the end of first response stage, temperature of outer propellant on the side is 397 K, and temperature of outer propellant on the bottom is 359 K. After 30 s of fast cook-off, the temperature of propellant began to enter a stable rising stage. The temperature rise rate of outer propellant on the side is 1.55 K/s. The temperature rise rate of outer propellant on the bottom is 1.09 K/s. The temperature of inner propellant is not high enough to cause self-ignition. It shows that radial heat transfer is not the main factor that causes the motor to self-ignite in fast cook-off.