Abstract: In the paper,a dual beam side shot virtual cathode oscillator（SSVCO） model has beenestablished and simulated by a 2.5-dimensional,fully electromagnetic, relativistic CIC particlesimulation code WGCF。The simulations （including single beam case）indicate:（1）the maximum averageefficiency occurs for a fixed distance of the two anodes when the injected beam current J₀ is 1.2～2times as large as the critical current J_c;（2）there is an optimum distance where the maximum efficiencyis reached for a given ratio of J₀/J_c;（3）as compared with single beam case,the microwave modesare purer,no direct current fields are coupled to output window and the central frequencyexpands wider,（4）oscillations still exist for J₀～J_c,but d.c. fields are dominant.Besides,thepower distribution functions with modes or frequencies and a new absorbing boundary condition have been derived.