The piston in reciprocating engine would be badly ablated under severe knock. However, the mechanism of the detonation-induced thermal ablation of piston is still unclear. A detonation bomb device (DBD) was used to measure the thermal load of piston under detonation. A test specimen mounted on the detonation bomb acts as a piston to bear the detonation load. Transient thermal numerical analysis was performed using the finite element method. Temperature of the specimen and in-cylinder pressure were collected synchronously. A method for estimating wall heat flux under detonation was proposed. Results showed that the heat received by the specimen accounts for about 20.9% of the total heat released by the mixture in this research. Under continuous detonations, the heat of the surface layer could not be conducted to the interior in a short time, leading to a rapid rise in surface temperature. The overall temperature rise of the specimen limits the heat dissipation of the specimen surface layer, resulting in the specimen being ablated by the over-temperature and over-pressure. Piston thermal ablation by detonation is verified and reappeared in the detonation bomb. The thermal load of the piston is largest under theoretical equivalent ratio.