According to the application of radiant floor heating, system design, and laying status, this paper advances a design idea of using natural cooling source such as ground water or evaporation cooling water for ‘free cooling’. Based on the above, a mathematic model of low-temperature radiant floor heating/cooling is constructed. This paper studies the matching of radiant floor’s cooling capacity and building load, which is in the radiant floor cooling situation that meets thermal comfort of occupants, and in the precondition of fixed parameters such as supply and return water temperature, difference in temperature, flux, existing pipe distance, floor board material and others. In the next place, the change range of indoor air dew point temperature is analyzed in different indoor and outdoor environment operation situation. Combined with the model mentioned above, in the floor non-dewfall condition, the application scopes and optimal values of correlative design parameters are suggested. Finally, this paper considers the synthetically conditions of human thermal comfort and non-dewfall of floor, and then puts forward the feasibility suggestion and applicability project of summer cooling in existing radiant floor system.