The invention discloses a method for acquiring slack-free power flow based on pyramid approximation, comprising the steps of: establishing a power flow model of a power system based on a branch powerflow model and power system parameters; and splitting and standardizing an initial sub-model between line power and voltage current, and obtaining a sub-model between line power and voltage current having a conical surface as a geometric pattern; performing a pyramid approximation on the conical sub-model between line power and voltage current to obtain a slack-free linear power flow model; and successively subjecting the slack-free linear power flow model to rotational symmetrization and linearization processing to obtain a slack-free linear power flow model with integer variables reduced. The method converts the nonlinear model into the linear model without using slackening processing, thereby ensuring the authenticity of the power flow model. Further, the reduction of integer variablesby the rotationally symmetric method can greatly improve the computational efficiency.