In present paper, a two-dimensional numerical model for the coupled heat transfer process in the solar collector tube was developed and the unified dimensionless governing equations were derived. The full field computation method was adopted to deal with the fluid-solid conjugated heat transfer problem. The effects of Ra numbers and tube diameter ratio on the heat transfer and fluid flow performance were numerically analyzed under the constant heat flux conditions. The distributions of flow field, temperature field, local Nu and local temperature gradient were examined. The results show that when Ra is larger than 105 , the effects of nature convection must be taken into account. Due to the effect of the nature convection, along the θ direction, the temperature in the cross-section decreases and the temperature gradient on the inner tube surface increases at first. When θ is near to π, the temperature and temperature gradients would present a converse variation. The local Nu on the inner tube surface increases along θ direction until it reaches a maximum then it decreases again. Due to the existing of the nature convection vortexes, the temperature distributions in some cross-sections present fluctuations along the r direction.