The evaluation of the sensitive process economic loss caused by event power quality disturbances such as voltage sag and short supply interruption is the key to make the power quality solution investment decisions. This paper, embarking from the event power quality disturbance level, the running states of the equipment and the parameter immunity time (PIT), studies the characteristics of the process running states when happened voltage sag and makes PIT as the bridge of the event power quality disturbances suffered by the user and the resulting economic losses. The process running state diagnosis and identification method are studied in detail when voltage sags occur. The quality loss function is used to calculate the interruption membership of the non normal non interrupted process state to quantify losses, an improved sensitive process economic loss assessment method is proposed. Finally, a semiconductor manufacturing enterprise which the author's team has investigated in the past is taken as an example to prove the correctness and feasibility of the proposed method. This work is supported by the Science and Technology Project of State Grid Corporation of China (No. SGRI-DL- 71-15-006).