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.