With the significant development of wind power, it has been inevitable for wind farm cluster to integrate into the system from the grid end, therefore, the power grid loss problem of large-scale wind power integration is highlighted. At first, we construct a power transmission system model with large-scale wind power integration, get the power grid loss calculation formula of the system, and induce the change laws of the power grid loss by the constraints of power transmission channel flow. And then, based on the structure of Gansu power grid, we do some simulations about power grid loss distribution by the constraints of power transmission channel flow, and verify the correctness of theoretical derivation.