With the development of quantitative remote sensing, regional evapotranspiration modeling based on the feature space has made substantial progresses. Among many remote sensing evapotranspiration models, accurate determination of the theoretical dry/wet lines remains a challenging task. This paper reports the development of a new method, named DDTI (Determination of dry line by Thermal Inertia), which determines the theoretical dry line based on the relationship between thermal inertia and soil moisture. The Simplified Thermal Inertia value estimated in the North China Plain is consistent with the value computed in the laboratory. Two evaluation methods, which are based on the comparison of the location of theoretical dry line and the comparison of Evaporative fraction, were used to assess the performance of the new method DDTI. The location of the theoretical dry line determined by DDTI is higher than the heat energy balance method, which is more reasonable in wet conditions. When compared with the in situ measurement of Evaporative fraction at YuCheng Experimental Station, the ET model based on DDTI reproduces the pixel scale EF with a RMSE(Root Mean Square Error) of 0.071, which is much lower than that based on heat energy balance method with a RMSE of 0.65. Also, the bias between in situ measurements and DDTI method is 0.056，which is ower than that between in situ and the heat energy balance method with a bias of 0.645.