The invention relates to a circuit semi-accommodation parameter estimation method based on reactive loss deviation, and belongs to the technical field of electrical power system control. The circuit semi-accommodation parameter estimation method comprises the steps that the state estimation value of a circuit measurement point is calculated through state estimation, circuit reactive losses are calculated through a measurement value and the state estimation value of the circuit measurement, and the reactive loss deviation is also calculated; a circuit with the reactive loss deviation larger than a set threshold value is set as a suspicious circuit.; according to the reactive loss deviation, semi-accommodation parameters of a circuit are changed step by step through a variable-step size integral method, and estimation and evaluation indexes after the parameters are changed are obtained through state estimation calculation; the circuit semi-accommodation value corresponding to the optimal estimation and evaluation index is set as the semi-accommodation estimation parameter of the circuit. The circuit semi-accommodation parameter estimation method improves the circuit semi-accommodation parameter estimation accuracy.