The convective heat transfer characteristics between high temperature rocks and fluids in Enhanced Geothermal Systems (EGS) have been an important fundamental element of research on Hot Dry Rock (HDR). The thermal resistance of rock has an important influence on the convective heat transfer characteristics of fracture. In order to study its specific influence, two research methods, i.e., the theoretical analysis and the numerical simulation, were used in combination to study the heat transfer law between two parallel smooth flat plates by discussing the analytical solution and establishing the numerical model. The results show that the local Nusselt number Nu_x is constant and unrelated to other factors when the fluid velocity and the heat transfer boundary layer are fully developed. The local convective heat transfer coefficient h_x is only related to the fluid thermal conductivity k and the fracture opening e, but independent of other factors. Nu_x is 8.235 when the heat flux at the upper and lower flat wall is constant, and 7.54 when the wall temperature is constant. Then, several sets of convective heat transfer models of rock fissure with different thermal resistance were constructed. Thereby, it is found that the convective heat transfer coefficient h increases when the thermal resistance of the rock increases and the inlet section of the temperature field are extended. The length of the rock significantly affects the portion of the inlet section, which in turn affects the magnitude of h. Besides, h decreases with the increasing length. When the rock is long enough, the portion of the inlet section is small enough, and the parameters other than k and e have little effect on h. The rock commonly used in the laboratory is 100 mm in length, while the length of fractures in typical EGS is of the meter level. Therefore, it is recommended that special attention should be paid in the lab experiments to the influence of rock length on the convective heat transfer characteristics of fractures.