Considering that there is a lack of a comprehensive and universal method for assessing the influence of provincial power grid with large-capacity back-to-back VSC-HVDC connection, an influence assessment method is proposed in this paper. Several aspects such as analysis of static security, short circuit current level, small signal stability and transient stability are included in the method. Interaction between VSC-HVDC and AC system is emphatically considered. Taking back to back VSC-HVDC connecting Chongqing and Hubei power grid as an example, the operation characteristics of Hubei power grid are assessed with Power System Analysis Software Package (PSASP). According to assessment results, the corresponding reinforcing project and security control measures are proposed with the consideration of power grid planning and operation requirements. The analysis shows that the influence assessment method proposed in this paper can effectively guide the planning and dispatch control of power grid and ensure the safe and stable operation of the power system. This work is supported by National Basic Research Program of China (973 Program) (No. 2014CB247400) and National Natural Science Foundation of China (No. 51577075).