The fast cookoff of TATBbased PBX explosive with 2 mm thickness steel casing under solid propellant firing in a welldefined scenario such as failed launch of a missile was investigated. In addition,temperature distribution of the TATBbased PBX explosive under solid propellant firing was numerically investigated. Temperature rising was recorded and the heat flux of the explosive surface was calculated with the experimental data and Duhamel superposition principal,the average heat flux on the explosive surface under the solid propellant firing is 19.1 kW·m^-2. Ignition time of TATBbased PBX explosive under the solid propellant firing is 95 s. The experimental results show that fast cookoff of TATBbased PBX explosive under solid propellant firing will not develop into deflagration or detonation.