The invention relates to an electric system available transfer capability calculating method taking regard of a demand side response. The electric system available transfer capability calculating method comprises reading demand side response quotation, generator set quotation and system data; establishing a double-layer optimization model for active power distribution in dependence on the demand side response quotation and the generator set quotation, obtaining the initial running state of the system, i.e., the existing power transmission protocol; wherein the double-layer optimization model comprises an upper layer model and a lower layer model, the upper layer model comprises an optimization target function and two constraint conditions, and the lower layer model comprises an optimization target function and three constraint conditions; and employing an available transfer capability calculating model based on optimal power flow and taking the maximum value of the active output increment of all generating nodes in a generating area as a target function, and employing a DC optimal power flow model comprising a target function and constraint conditions. According to the invention, on the system initial operating points, the active load and generator output of each node is economically dispatched in dependence on the demand side response quotation and the generator set quotation, the power grid power flow distribution is optimized, and the power grid power transmission capability is improved.