The invention discloses an improved transmission loss equivalent load DC optimal power flow calculating method, and the method comprises the following steps: (1), introducing equivalent transmission loss grounding resistors to two ends of a line of an optimal power flow DC model, forming a transmission loss equivalent load model, and deducing the resistance values of the grounding resistors; (2),deducing an active loss formula of the line from an AC optimal power flow model; (3), carrying out the polynomial fitting of a trigonometric function term in the active loss formula, and eliminating avoltage amplitude terminal in the formula through the characteristics of a system; (4), carrying out the equivalent distribution of the active loss of the line to the equivalent transmission loss grounding resistors at two ends of the line, and forming an improved transmission loss equivalent load DC optimal power flow model. The precision and efficiency of the model is verified in a test, and the method effectively improves the calculation precision of the transmission loss equivalent load model, also guarantees the calculation efficiency of the model, and improves the practical value of thetransmission loss equivalent load model.