In the large voltage loss situation for middle and low voltage feeders， the power loss and voltage loss values calculated with the loss coefficient method are greatly different from their actual values. A line segment based method， i.e. improved loss coefficient method， for calculating the voltage and power loss is proposed to solve the problem mentioned above. By dividing a line into N segments， new loss estimation formulas are derived when the segment number tends to be infinite by using the related mathematical knowledge with the losses for each segment being obtained. By comparing the calculated results of three methods， i.e. loss coefficient method， improved loss coefficient method and finite element analysis， in different situations， it shows that the proposed improvement of loss coefficient method can greatly reduce the error， especially for the lines with large voltage loss.