Sensing of Nitrogen dioxide (NO₂) is of great importance for its pernicious and destructive impacts to both human health and nature environment. However, it still remains challenging to achieve the NO₂ detection with fast response, high sensitivity and good selectivity. In this work, two-dimensional (2D) ultrathin n-type SnS₂ nanosheets with Mo doping have been synthesized via a simple single-step solvothermal method and exploited for NO₂ sensing application. Phase and structural analyses confirm the homogeneous cation alloying of Mo (1% ∼ 10%) in the SnS₂. It is indicated that the Mo doping can elegantly tune the electronic structure of SnS₂ and promote the sensing process with negative adsorption energy. Especially for 3%Mo-SnS₂ nanosheet, the NO₂ sensing response at 150 ℃ has been enhanced around 23 times relative to the un-doped SnS₂ sample, together with fast sensing kinetics and good NO₂ selectivity. Interestingly, the n-type SnS₂-based sensor shows an abnormal response characteristic for the first time with resistance decrease at low NO₂ concentration, behaving like a p-type semiconductor. The P-N sensing transition can be regulated by varying the operation temperature and NO₂ concentration. Mechanisms for the enhanced sensing and abnormal phenomena are well explained based on density function theory calculations. Such P-N sensing switch in a single sensor here opens up interesting possibilities for the highly sensitive and selective detection of NO₂ with 2D metal dichalcogenides.