Airspace capacity is an important basis for the implementation of air traffic flow management measures. Especially in thunderstorm weather conditions, accurate prediction of the airspace operation capacity is the focus and difficulty during flow management tactical phase. The purpose of this study is to introduce a computational model that quantitatively evaluating the impact of weather on airspace capacity, and a method for applying the model at the operational level on flow management tactical phase, and to guide the development of collaborative decision support tools for meteorological and flow managers. The innovation of this study is to select five key meteorological indicators, including echo intensity, precipitation intensity, cloud top height, coverage and location trend, which the flow manager focuses on. Combined with the actual operation, the classification standard of index grade is further given, and the applicability of the model to different airspace structures is improved by using index weight. Through empirical research on typical scenarios, the accuracy and practicability of the model are verified, which shows high application value in flow management tactical phase.