This paper presents a kind of composite customer damage function and a estimation method to the customers’ outage costs based on the reliability. To build the function, the outage loss of customers in each type in the time of peak load is acquired based on the method of customer survey, and the factors are taken into account such as the energy consumption ratio and load rating of various users. The estimation method is proposed based on the effects of the reliability to users made by the elements in the different positions on the line. This method can provide a reference for the engineers to make economic decisions about network planning and construction. Finally, the test system of IEEE RBTS is used to illustrate the feasibility and effectiveness of this method.