The principle of twice synthesis comparison method is introduced in this paper.A novel approach based on time series analysis for analyze tan δ and C obtained from on-line monitor of capacitive equipment is presented.The threshold value of the first warning is calculated based on mathematical statistics and is confirmed by the number of the successive warnings.The second warning is made on the threshold amended based on circumstances condition so that the disturbance caused by surroundings and monitoring system can be effectively reduced.Analysis results of experiment data have verified its effectiveness.