Accumulation variable method is an efficient method for online active power dispatch. The relation between available generating capacity and ramping rate of load over whole system is considered in this method. However， since it ignores the transfer capability constraints， it is not applicable for a grid with several sub-control areas. To solve this problem， based on the accumulation variable method， a modified method is presented， in which， the constraints of transfer capability between different control areas are included. A two-step algorithm is proposed to solve this model. In the first step， an optimal solution is found without considering constraints of transfer capability. In the second step， a correcting method is employed to eliminate the transfer capability violations. Results of case study validate the proposed algorithm.