Abstract: For keeping power systems safety and ensuring the cross-regional electricity trading,a calculation method of available transfer capability（ATC）for cross-regional interconnected power grid is proposed.Compared with other methods,only boundary bus voltages and increment loads at buyer bus are sent to/received from other areas.Firstly,an external network equivalent,which is suitable for ATC computation,is built to decompose the inter-connected power grid.Secondly,fictitious slack buses and auto-adaptive power injections are set that each area can run its own repeated power flow.Thirdly,distributed power flow calculation is used to correct power flow in every area,which prevents a larger mismatch happening in areas’power flow calculations.Finally,the ATC for the cross-regional interconnected power grid is computed based on ATC definition.According to simulations on IEEE 118-bus test system,the method is proved to be effective and accurate.