In the flexible DC distribution power system, if the converter station is connected to the AC grid via a connecting transformer, the insulation damage between the connecting transformer and the converter valves will cause a grounded fault on the AC side of the converter station. The influence of this grounded fault and the protection strategy is worth researching. First, the equivalent common-mode circuit model of the components in the flexible DC distribution power system is analyzed. Subsequently, the equivalent common-mode circuit model of a flexible five-terminal DC distribution power system with different grounding methods is built. The calculation formulas of fault fundamental frequency common-mode current and voltage after a converter AC side single-phase grounded fault occurs are obtained. Then, considering various resonances that may exist in a DC system, a protection strategy based on a converter DC side fundamental frequency common-mode voltage, current and power direction to identify the single-phase grounded fault on the converter AC side is proposed and verified on the PSCAD platform. This work is supported by National Key Research and Development Program of China (No. 2017YFB0903100).