SnS2 field effect transistor (FET) has been fabricated based on an exfoliated SnS2 flake. The effect of vacuum and sulfur-vapor annealing on the electric and optoelectronic properties of SnS2 FET have been investigated in detail. The experimental results indicate post-annealing, especially the sulfur-vapor annealing, has a strong impact on the electrical and photoelectric properties of SnS2 FET. The decreasing of source-drain current with annealing, the shift of threshold voltage and the exponential function of photocurrent varying with light intensity all exhibit a close relevance with surface states in SnS2. The intriguing characteristics can be well explained by sulfur-vacancies-related trapping mechanism. These results not only advance the current understanding of the generation, trapping and recombination behavior of photoexcited carriers in two-dimensional materials, but also stimulate the potential applications in optoelectronic devices based on SnS2.