Abstract: In this article, the compressing and forming mechanism of hot spot in the implosion of shock ignition is analyzed using the method of simulation. Firstly, the implosion of conventional central ignition is discussed. The hot spot experiences shock compression and inertial compression in conventional central ignition, and ignition is achieved mainly by inertial compression. Then, the implosion of shock ignition is analyzed in detail. Shock ignition is not a two-step scheme that the assembly of the fuel is separated from ignition. Ignition is simultaneous with assembly. The ignitor shock participates in the assembly and has little direct impact on the hot spot. Ignition is still mainly achieved by inertial compression in shock ignition. Finally, the physical mechanism of the hot spot pressure improvement is analyzed according to the hot spot pressure scaling law of inertial compression and the impact of shock collision on the shell. The improvement of hot spot pressure results from the enhancement of shell density due to the collision of the ignitor shock and the return shock in the shell.