Pressures around tunnels are the priority of the design and construction of underground excavations. One of the designs for Badaling underground station of new Beijing-Zhangjiakou intercity railway utilizes a large span single arch excavation method. In this design, the excavation span is approximate 45 m. Traditional methods to predict the rock mass pressures around tunnels are not suitable for the case in Badaling underground station because of the large span. Instead of the traditional methods, numerical simulations can provide a more accurate result but with a more complex and non computation-efficient process. Comparing the differences between various traditional calculation methods and numerical simulation method, a rapid estimation method of rock mass pressures for deep and large-span underground excavations is proposed based on the Q system. The newly proposed method introduces the factor of span into the original Q system. Comparison of surrounding rock pressures between estimated and monitoring results of underground structures with different spans shows that this method could provide engineers and designers with a quick way to predict rock mass pressures in the design and construction stage of underground excavations as well as researchers with a reference guide.