In order to improve the operating level of largescale power grid and contingency treatment skill of operators，a method for power flow transfer analysis and power flow control based on equal-phase line is proposed.By using the basic viewpoint of DC flow，a largescale power grid can be divided into the transmission parts and acceptance parts，and the complicated structure of the grid is simplified to the upper and lower passages.General conclusions are drawn through analysis that the transfer between transmission paths on the equal-phase lines close to each other is rather great;the areas on the lower equal-phase lines（the lower power grid）can provide first-aid to the areas on the upper equal-phase lines（the upper power grid）should contingencies happen.Examples in a provincial power grid in East China power grid show that the general conclusions of the equal-phase line theory have strong applications in complicated grids，while the method can provide a basis for post-contingency power flow transfer analysis and adoption of rational control measures.