Abstract: The internal system generated electromagnetic pulse (SGEMP) effects induced by photoelectrons emitted from cylinder top surface is simulated by the 3D PIC code. First for validation, the results are compared to that from literature. Subsequently, the internal 3D SGEMP effects of the cylinder is studied by this code, the simulated results show the emitting current is 20 A, intense of electric field could get 150 kV/m and magnetic field is 3.0×10⁻⁵ T when the blackbody X-ray of temperature 2 keV and fluence 100 J/m² perpendicularly irradiates the side of cylinder cavity with height of 4.5 cm and radius of 7.5 cm. The space distributions of electrons and electric field are studied elementarily for different X-ray fluences.