Abstract: The 300 MeV proton and heavy ion accelerator is an important component of the SESRI(Space Environment Simulation and Research Infrastructure), and the study on slow extraction dynamics is the key part of the accelerator. The third order resonance and RF-Knockout(RF-KO) scheme is adopted to provide quasi-continuous beam in 2~8 s for slow extraction system. 3-bump are used to adjust the spiral step and extraction angle at the electrostatic septum. However, the horizontal tune can be reduced due to the bump orbit, and the stable area in the phase space is shrunk during the extraction flattop. Simulation results show that the bump orbit produced in the dipole and sextupole magnet will reduce the horizontal tune, and a large number of particles are extracted at the initial extraction stage. Therefore, the spill structure should be analyzed and optimized based on the self-written particle tracking program SESP. The extracted beam uniformity is improved by modulating the amplitude of the extraction exciter.