A simulating model was used to simulate and improve the thermal performance of solar greenhouses. Simulations with the weather conditions of Xuzhou, Jiangsu Province show that the greenhouse with clay brick wall (0.6 m thick) insulated with polystyrene boards (0.1 m) and the greenhouse with soil wall (3m thick) perform better than the one with hollow concrete block wall (0.6 m thick). The lowest air temperature increases with thermal resistance and thermal inertia index of north wall. The thermal performance of the solar greenhouse with hollow concrete block wall can be improved by increasing the density and reducing the thermal conductivity of north wall. The soil wall can be conceptually divided into three layers from inside to outside: energy-storing layer, thermal stable layer, and thermal insulating layer. Thermal stable layer shrinks with the reduction of the size of north wall and it would disappear when the wall is reduced to a certain size. A trapezoidal soil wall with a top of 0.5 m and a base of 1.7 m appears to be a proper design if the air temperature is to be maintained above 10 degrees C under the weather conditions of Xuzhou, Jiangsu Province of China. (C) 2014 Elsevier B.V. All rights reserved.