High error in calculating the heat transfer of floor heating system can be introduced by existing models, considering the heat transfer of the floor slab and different temperature between interior wall and exterior wall. A modified calculation model of heat transfer of floor heating system was proposed, analyzed the influence of the interior or exterior wall and the floor slab, and discussed the disadvantages of zero heat-flux models. The affected factors of heat transfer of floor heating system were studied by using the new model. The results show that, (1) Certain temperature is the best boundary condition between interior wall and exterior wall, for the case the temperature are different between interior wall and exterior wall, and high error can be introduced by zero heat-flux model. (2) Heat radiation is the best boundary condition of under surface of floor slab. (3) The temperature growth of floor surface decrease, with increasing the diameter of pipeline. The temperature periodicity distribution of floor surface more clearly, with increasing the distance between pipelines or hot water temperature. The length of periodicity increases, with increasing the distance between pipelines. The swing length of periodicity increases, with increasing the hot water temperature. It is also found that it is important to floor surface layer material.