In the past, the structures of conventional direct current (DC) and flexible DC transmission projects were end-to-end or multi-terminal radiation. The corresponding stability control system uses the converter electric quantity and pole emergency switch off (ESOF) signal as the criterion of pole blocking. It is not suitable for the application of flexible DC power transmission engineering with ring structure, because the DC converter blocking failure cannot be identified reliably. According to the development experience of stability control system of Zhangbei voltage source converter based high voltage direct current(VSC-HVDC) transmission project, a new improved method of DC pole blocking detection is proposed in this paper for stability control. This criterion adopts the characteristic signal of DC control and protection as the main judgment object when DC light load is applied, and adopts the characteristic signal of DC control and protection combined with the converter electric quantity as the main judgment object when the DC power is above 10% of rated power. The improved detection is applicable to both HVDC transmission projects and ring structure flexible DC transmission projects. At the same time, the calculation method of stability control measures of Zhangbei VSC-HVDC transmission project is proposed, which provides a reference for the application of subsequent flexible DC transmission projects and HVDC transmission projects. Theoretical analysis and real-time digital simulation system (RTDS) experiment verifies the reliability of the criterion.