A coupled thermal lattice Boltzmann model was adopted to simulate the natural convection in an annulus space with an internal slotted circle. The influences of the Rayleigh number and slotted degree on the flow and heat transfer in the annulus space were discussed. The results show that, with the increase of Rayleigh number, the natural convection in the annulus space will appear strongly instable and the flow regime transition is also multiple. There are four kinds of flow patterns:steady two-eddy flow, four-eddy flow, oscillation between two-eddy and four-eddy flow and oscillatory two-eddy flow. In addition, the influence of the variation of slotted degree on the natural convective heat transfer in the annulus space is significant. With the increase of the slotted degree, the fluid is more likely to form a multi-eddy pattern's flow, which in turn increases the fluid disturbance and enhances the flow instability. The results also indicate that increasing the slotted degree can enhance the natural convective heat transfer. But the slotted degree is not the larger the better, there is a critical value.