Combined with 1 000 kV AC transmission pilot project in China， a new automatic generation control（AGC） performance assessment index suitable for the UHV AC tie-line active power control， named responsibility degree index（RDI）， is provided to correctly evaluate the responsibilities of North China power grid and Central China power grid for the power fluctuation on the UHV AC tie-lines as well as their contributions to the fluctuation. The application of RDI to the commissioned UHV AC tie-lines show that it has achieved the favorable control performance. On this basis， a new AGC performance standard， named Standard T， is further proposed based on RDI. At last， considering the characteristics of the soon completed UHV synchronous North， Central and East China power grid， a preliminary discussion is made on the practical applications of CPC， CPS and Standard T.