An optical pump terahertz probe measurement was performed on monolayer WS2 and h-BN/WS2/h-BN vertical van der Waals heterostructure to investigate the effect of h-BN layer on the ultrafast dynamical process of photo excited charge carriers and the complex conductivity. Based on our measurement, we find that the initial fast carrier relaxation process which is dominated by the formation of charge neutral excitons is similar in the monolayer WS2 and h-BN/WS2/h-BN heterostructure. This result indicates the effect of h-BN layer on the exciton formation process can be simply omitted. Compared to initial relaxation process, the following relaxation process, which is dominated by the Auger process and impurity or defect center assisted non-radiative process, of h-BN/WS2/h-BN heterostructure is around three times longer than that of monolayer WS2. We attribute the longer lifetime of h-BN/WS2/h-BN heterostructure to the protection of h-BN layer, which inhibits the formation of oxygen impurity and S-vacancy defect center in the WS2 layer. We further derive the complex conductivity of both monolayer WS2 and h-BN/WS2/h-BN heterostructure from our terahertz spectroscopy measurement and analyze our results using the classical Drude-Smith model.