In order to meet the specification requirements for the draft pipe inlet pressure, it is crucial to select a rational guide vane closure procedure in the hydraulic transition process of large capacity generation unit in high altitude areas, which is one of the most common and economic measures for the safety operation of the hydropower station. In this paper, taking a hydropower station with large capacity generation unit in a high altitude area as the study object, we adopted characteristic line method to calculate the closure procedures using transition process simulation software. The results show that choosing the appropriate broken-line closure procedure of the guide vane can effectively improve the draft pipe inlet pressure of such hydropower stations, so as to meet the control standard which is revised based on the high altitude. The research plays an important guiding role in the application of specifications to the draft pipe inlet pressure control of hydropower stations with large capacity generation units in high altitude areas.