The present invention belongs to the power grid network loss analysis calculation technology field in the power system, and especially relates to a calculation method for sensitivity of transformer substation active load to regional power grid active transmission loss in the power grid online theory transmission loss calculation. The method comprises: on the basis of real-time load flow calculation result, solving the sensitivity of each bus voltage to the total transmission loss of the region, calculating the transformer substation including low-voltage side equivalent load in the regional power grid, calculating the sensitivity of the active load of each equivalent load in the transformer substation to the bus voltage, superposing the sensitivity of each bus voltage to the total transmission loss of the region and the sensitivity of the active load of each equivalent load in the transformer substation to the bus voltage to obtain the sensitivity of the equivalent load for the regional power grid active transmission loss, and finally, obtaining the arithmetic average value of all the equivalent load sensitivities included by the transformer substation, namely the sensitivity of the transformer substation active load to the regional power grid transmission loss. The calculation method for sensitivity of transformer substation active load to regional power grid active transmission loss can provide the sensitivity sequence of the transformer substation active load so as to guide transmission loss reduction and provide an auxiliary decision for real-time loss reduction.