The gas-sensing characteristics of doped SnO2 materials for ammonia were investigated in this paper. A good formula of ammonia sensitive materials has been achieved. The phase of sensitive material was characterized by X-ray diffraction. The gas sensor of indirect heating that was made by this sensitive material had high sensitivity to ammonia gas and stability. Infrared was used to study the reacting products of NH3 on the surface of the material. The sensitive mechanism of sensor was analyzed and discussed.