In order to solve the large computational problem of transient stability preventive control and total transfer capability（TTC） calculation in large scale grid， a novel cluster computing method which can calculate online is proposed in this paper. Firstly， generators participation factor is calculated using extended equal-area criterion（EEAC）. Secondly， control performance index comes from control cost divided by the sum of the participation factor which weighed by margin of fault. Thirdly， adjustment schemes which come from the result of head and tail pair based on sorted index of generators are computed in parallel. In the TTC calculation，the generator output is changed on the principle which bigger participation factor gives priority to increasing in critical cluster and smaller participation factor gives priority to reducing in remaining cluster. Finally， every possible power level is computed in parallel. The data results of North China， Central China， and Sichuan-Chongqing interconnected power system prove that this method can meet online computing requirement of large scale grid rapidly and effectively.