In order to reduce the time of large power system power flow computation, the study of power flow computation method for reducing power grid is much more significant. But, the existing method of Ward is used to the reduce grid and different result as the original grid is got. Aiming at this condition, an improved DC power flow computation method is presented in reduced power system. Based on the basis of present study results, the DC power flow computation method based on power transfer distribution factor is deduced. The Ward technology is used to reduce the power grid. Combining with the two parts, the power flow computation method for reduction grid based on power transfer distribution factor is given. Simulation results show that experimental result has high precision, and it can be applied in the power flow computation for the reduced power grid and power flow forecast and so on.