In order to enhance the efficiency and reduce the manufacturing cost of steam turbine, combined main steam valve consisting of main stop valve and control valve in the same casing has been developed. Combined main steam valve with improved flow path at internal structure will reduce pressure loss. A downscale model test on air condition was conducted to verify pressure distribution characteristics. Experimental result and numerical result obtained by computational fluid dynamics (CFD) showed reasonable agreement in pressure loss and static pressure at each flow point within the design operation range. Main stop valve (SV) and control valve (CV) vibration characteristics tests were performed simultaneously for the stability verification of governing operation from start up to full load. We confirmed that CV aerodynamic added damping ratio was positive under all measured condition of CV stroke and pressure ratio, and self-excited vibration was not generated. We also confirmed that pressure fluctuation acting on main stop valve body was sufficiently small within the operation range. Meanwhile, in the case where pressure ratio or CV stroke deviated from the operation range, pressure fluctuation around main stop valve increased. Based on CFD result and detailed analysis of experimental result, it was found that steam flow along valve seat separates periodically, which revealed the mechanism of increasing pressure fluctuation around main stop valve. By reflecting on these results, the reliability of new combined main steam valve has substantially been enhanced.