Abstract: Single Event Upset (SEU) induced by atmospheric neutron is harmful for near space craft, and is studied by numeric simulation in present paper. Static Random-Access Memory (SRAM) device which has regularly arrayed cells is selected as the simulation object. Using Rectangular Parallelepiped (RPP) Model as the basic model and extracting key parameters from SEU experiment data of heavy ions, sensitive volume model of SEU due to nuclear reactions between incident neutron and device materials is established. Simulation and research show that the secondary heavy ions resulting from the high energy neutrons can transfer approximate 4 microns. Consequently, to simulate the SEU events thoroughly, an extra volume beyond 4 microns from the sensitive volume is taken into account. Modeling and simulation are performed for HM62256 and HM628128 SRAM devices respectively with the help of GEANT 4 software package. The secondary particles in and near the sensitive volume contributing to SEU are considered. Differential spectrums of the deposited energy in the sensitive volume by the secondary particles are calculated. Eventually SEU events due to the secondary particles which deposited different energy in the sensitive volume are summed up. The results of the simulation agree well with those of ground experiments performed with 14 MeV neutrons and high energy protons.