Substation capacity-to-load ratio is an important index of distribution network planning， and existing guideline indicates that capacity-to-load ratio can be lower when subordinate distribution network is strong；however， there are no quantitative analysis methods. This paper proposed a quantitative analysis method for the relationship between capacity-to-load ratio and subordinate medium voltage distribution network based on the total supply capability （TSC） theory. TSC theory can be used to calculate the maximum load supply capacity of a distribution network under the condition of meeting the “N-1” security criteria， with taking the substation transformer transfer in transformer substation and the subordinate network transfer into account. This paper analyzed the capacity-to-load ratio data with different network strength， and explored the relationship between capacity-to-load ratio and network strength， as well as the range of capacity-to-load ratio through changing feeder capacity， connection location and scale. Finally， the quantitative conclusion of lower capacity-to-load ratio and its recommendations were proposed， which could enrich and develop the existing planning guidelines.