With the development of the active distribution network and the distributed energy, the links between the transmission grid and distribution network are more and more close and complex. The reliability of the transmission grid should be fully considered when the distribution network reliability assessment is studied. This paper presents a method to quantify the reliability of the transmission grid, in order to enhance the practicability of the reliability assessment of the distribution network. First, this paper presents the concept of maximum load capacity that shows the maximum power supply capacity of the transmission grid to supply the downstream distribution network, and then the transmission system can be equivalent to the power supply node of the distribution network. Secondly, Monte Carlo simulation and optimization techniques are used to establish the maximum load capacity evaluation model. Finally, RBTS transmission test system is used to verify the effectiveness of the proposed method of quantifying the reliability of the transmission grid.