In order to determine the structure of valve pipe and descending height of valve core, fluid flow of the filling valve of BIB was analyzed and calculated. Flow field analysis was conducted for the structure of bag-in-box valve by Fluent software to find out the stress concentration area based on the distribution of velocity field and pressure field and optimize the structure. Then analog simulation was carried out after the valve core descended for different heights by the Fluent software to find out the best height according to the change in velocity field and pressure field. When θ1 and θ2 were at 30°, the valve core descended for 3mm, and materials flowed at the best conditions in the valve pipe. Therefore, to get the descending height at 3 mm, θ1 and θ2 are set at 30°.