Silica aerogel as highly efficient thermal insulation material has a great application potential in many fields. For better understanding of the mechanism of its super insulation performance, different physical and mathematic models were proposed by researchers. However, most of them can not describe all the details underlying, especially the contribution of gas and solid gas coupled conductivity. Besides, the separate contributions are difficult to measure by experiment. In the present study, non-equilibrium molecular dynamic (NEMD) simulations were conducted on a typical unit of aerogel nano porous structure. The solid skeleton was generated first to investigate the solid conductivity affected by the inter-particle contact conditions and defects concentration. Then the gas phase was added into the system with solid fixed to obtain the gas conductivity without coupling effect. Meanwhile a system with the same composition but no constraint applied was modeled to evaluate the solid-gas coupled thermal conductivity. The results show that the decreasing contact length and increasing defect concentration lead to lower solid conductivity. Moreover, the solid-gas coupling effect is not negligible when the necklace-like structure of aerogel is loose. These findings provide some details of the heat transfer mechanism of the nano porous structure at atom level and may help to the design of more insulating aerogel material.