Line loss rate calculation methods currently used by power distribution service providers are outdated and can no longer meet the requirement of line loss calculation needs today. Incorporating the actual power system situations， a quantitative calculation method is proposed. Comprehensive considerations of various factors that can affect the fluctuation in the line loss rate are incorporated into the method. These factors include the meter reading method， load fluctuation， and cross-border electric quantity， etc. The actual electricity consumption for the low tension users， special transformer users， and large industrial users are separately calculated to get the actual total electricity consumption. The monthly actual line loss rate is then obtained by traditional formula for line loss rate calculation. MATLAB simulation results show that the method proposed has higher reliability and precision in the practical applications for calculating line loss rate. The results might be widely used in guiding the work on power network planning and loss reduction.