Main steam isolation valves are used in nuclear steam supply systems. Their sole function is to close, in the improbable event of a rupture in the steam piping. Computational analysis of the valve enables the study of system responses that are otherwise difficult to obtain. In this paper, transient behavior during the valve closing was simulated by static analysis with five typical runs of valve openings. One-way fluid-structure coupling simulations were conducted to investigate the flow patterns and to estimate the deformation and stress of the valve. Results show that the main pressure drop of flow occurs in the throat path. All the deformation and stress of the valve are within allowed limits to ensure safety.