The large diameter disc-type special control valve (i. e. special valve) is one of the main control valves which control timely temperature and pressure of the air intake system to the temperature and pressure of flight environment for altitude test facilities.In order to improve control model precision of the special valve flow coefficient,the flow characteristics were studied based on Fluent software simulation analysis， and the influences of opening of special valve (10°,20°,30°,40°,50°,60°,70°,80°,and 90° (full open)) and pressure ratio (0.1,0.2,0.3,0.4,0.5,0.6,0.7,0.8,and 0.9) on internal velocity and pressure of special valves were studied.The results show that the velocity and pressure are symmetrically distributes along the stepless adjustment disc of special valves,and the air velocity gradually increases with an increase of opening.When upstream pressure of the valve is 200 kPa 〖HJ1.75mm〗and the pressure ratio is 0.1,the inlet velocity is 72.92 m/s in the opening 90°,which is about 9 times of the opening 10°.At the same time,when the opening is constant,the inlet velocity firstly remains constant then decreases with an increase of pressure ratio,there is a critical pressure ratio.The critical pressure ratio is about 0.5 at opening between 10° and 50°,and it is about 0.6 at opening between 60° and 90°.Finally,the flow resistance coefficient decreases with an increase of opening and pressure ratio,and it decreases dramatically when the opening varied between 10° and 40°.Especially， the flow resistance coefficient decreases 88% with an increase of opening from 10° to 30°.