Southwest China Power Grid which is un-synchronized interconnection with Central-North China Power Grid is calculated and analyzed after the commissioning of back to back VSC-HVDC between Chongqing and Hubei power grid. The operation characteristic of Southwest China power grid after asynchronous interconnection is studied, including small signal stability, rotor angle stability, and frequency stability. Results show that with Chongqing and Hubei back to back commissioning small signal stability varies and the weekly damped oscillation excited by faults around Jianshan substation becomes to be new constraint of sending of hydropower. Transient rotor angle stability is improved and is no longer the main factor to restrict the power transmission. It should be noted that frequency stability problem is prominent, and there is a serious risk of ultra-low frequency oscillation due to the negative damping of hydro generators. Finally, corresponding suggestions and measures are put forward for the above risks. The study could be technical reference for the planning and operation of the Southwest Power Grid. This work is supported by Science and Technology Project of State Grid Corporation of China “Study on source-grid collaboration frequency control technology for large-scale energy bases with multi-DC and AC/DC transmission systems”.