Load shedding calculation is a key step in transmission system risk assessment. To overcome the shortcomings of the current load shedding model, such as its ignorance of reactive power flow and node voltage constraints, and the ignorance of the impact of load shedding priority on the reliability index, a hierarchical optimal load shedding model is proposed. Load is hierarchically cut from high to low according to the load shedding priority, and the principle of proximity and optimization is taken into account. The nodes participating in the optimal load shedding calculation are limited within a certain range to reduce the calculation scale. The IEEE-RTS6 example shows that the proposed model could greatly reduce the operation times and power shortage expectations of important loads compared with the traditional method under the premise of ensuring accuracy. This is more in line with the actual system requirements, and to some extent reduces the calculation time. This work is supported by the Science and Technology Project of the Headquarters of the State Grid Corporation of China (No. 5500-202022072A-0-0-00).