The synchronous interconnection between Chongqing and Hubei power grid in China is planned to be unsynchronized by back to back VSC-HVDC. The operation characteristic of central China power grid should be realized with such un-synchronization. Short circuit computation and small signal analysis with Central China Power Grid as well as simulation of large disturbance within Sichuan power grid are undertaken to find the difference before and after unsynchronized interconnection is done. Results show that with the unsynchronized interconnection short circuit current decreased in adjacent three gorge area and low frequency oscillation modes have small variation while transient stability suffers serious deterioration. On the other hand unexpected islanding might happen near longquan substation or Enshi power grid. It is proposed to add one more line to the existing Longdou lines, Yiyu line and Enyu lines which are proofed to be effective against such stability deterioration and risk of islanding. This study could be referred to the Central China Power Grid planning, power grid operation and other similar HVDC transmission projects.