The object emissivity characteristics and the theory of heat radiation were studied to detect sand dust storm by remote sensing technique. The results show that the material nature of sand dust, ground surface and clouds is different when the sand dust happens. The ratio between the ground and sand dust in pixel images is different when the strength of sand dust changes. And these differences lead to the variations of material nature in pixels and object emissivity. At last, by a combination of theoretical and experimental researches, it was found that the degree of approach between ε 29 and ε 31 was different among cloud area, ground surface, weak sand dust and strong sand dust areas. And in a whole range of 0～1 of ε 31 , the order of ε 29 is cloud>strong sand dust>weak sand dust>ground surface. According to the results of the above, it was established a stable sand dust index (DSI) model to identify the sand dust strength . And the model can more accurately detect the range and intensity of sand dust after several rounds of sand dust examples.