How to explore the physical effect and characterize physical parameters in atomic scale crystals is still a challenging issue in modern optics. We detect the photonic spin Hall effect in monolayer MoS2 to study the atomic light-matter interaction. Due to the sensitive dependence of the spin-Hall shifts on the optical constants of MoS2, we propose this fundamental effect as a metrological tool for the parameter characterization. Combined with the weak-value technique, the experimental results show that the complex refractive index can be extracted by treating the atomically thin MoS2 as a homogeneous slab with an effective thickness. However, modeling the thin crystal as a surface current can determine the optical conductivity with a high measurement resolution. We believe this fast and precise measurement of optical parameters can provide a reliable approach to investigate optical properties for transition metal dichalcogenides.