Spontaneous imbibition experiments with two ends open (TEO) boundary condition showed that oil production from each open face of core is asymmetrical while the invasion of water is symmetrical. Investigating the asymmetry characteristics of oil production is helpful to understand the imbibition displacement mechanisms. In this paper, a mathematical model considering the difference in capillary back pressure for TEO imbibition is established by assuming piston-like advance of the imbibition front. Based on the model, the reason for asymmetry in oil production is discussed and the effect of the viscosity ratio, relative permeability ratio, average capillary back pressure and the difference in capillary back pressure on the asymmetry in oil production is investigated as well. The simulated results show that asymmetry in oil production depends on the ratio of the difference in capillary back pressure to the pressure drop in oil between the imbibition front and the open face of the core. As capillary driving pressure dissipated in oil is very small, a small difference in capillary back pressure will cause a significant asymmetric production of oil. Furthermore, the asymmetry in oil production decreases with increasing viscosity ratio ( and relative permeability ratio and increases with increasing average capillary back pressure and the difference in capillary back pressure. This work gives us a comprehensive insight into the spontaneous imbibition with TEO boundary condition.