Tungsten disulfide (WS2), as a representative layered transition metal dichalcogenides (TMDs), is expected as a promising candidate for high-performance NH3 sensor at room temperature. Unfortunately, the common WS2 based NH3 sensors are difficult to recovery at room temperature, which severely limits its application. Hence, how to improve recovery has become an urgent problem to be solved. Herein, we prepare five types of WS2 nanosheets with different layer numbers from bulk to monolayer, and find that the recovery time of NH3 gas sensor is rapidly linear shorten as the number of layers decreasing. Through the first-principles calculation of the interaction between NH3 and WS2 substance, the different binding energy between ammonia and the surface (-0.179 eV) and interlayer (-0.356 eV) of layered WS2, as well as the different electron transfer way, should be responsible for the difficult recovery rate of various WS2 samples. Therefore, reducing the number of layer of WS2 is a promising approach to speed up recovery. Based on this conclusion, we successfully prepare a fast recoverable ammonia gas sensor based on single layer WS2, which exhibits exciting fast recovery within 271.9 s at room temperature without any condition. Moreover, our work also can act as a reference for other gas detection of TMDs based gas sensor to improve the gas performance at room-temperature. (C) 2017 Elsevier B.V. All rights reserved.