Asynchronous line loss statistics of smart distribution network due to automatic meter failure and data missing are confusing. To solve this problem, a new method for synchronous line loss statistics based on power load or electricity consumption forecasting is put forward. Through data mining, this paper presents a monthly electricity forecasting method based on the result of annual electricity by grey model and quarterly and monthly hierarchic proportion. The average relative forecasting error is only 1.94% which indicates that this method is simple and effective and can be widely applied in electric department. And the forecasting results coupled with daily power supply ratio are applied in synchronous line loss statistic, which improves the inconsistency of power supply and electricity consumption and makes them synchronous. It has actual meaning for monthly line loss analysis.