It is a requirement of estimating voltage sag loss and providing a treatment scheme to know the impact of voltage sag on the industrial process. This paper considers the tolerance of voltage sags at the equipment/process level, and presents an evaluation method of load loss rate to quantify the harm of voltage sags. First, to solve the problems of obtaining process parameters and low accuracy, an equivalent response model of process parameters based on physical structure is established. The non-electrical process parameters at the process level are equivalent to the voltage values, and the characteristics of sag are used to describe the tolerance characteristics of equipment and industrial processes. Then, the load loss rate evaluation method based on Larsen reasoning is proposed. Taking the sag characteristic quantity as the antecedent and the load loss rate as the aftereffect, the membership functions are constructed. The evaluation of the load loss rate based on the monitoring data is realized, and the dependence of the evaluation result on accurate information is reduced. Finally, the method is applied to evaluate the load loss rate of a petrochemical enterprise in Hainan Province. The results show that the method proposed in this paper is correct and effective. This work is supported by the Science and Technology Project of China Southern Power Grid (No. 073000KK52190007).