Existing piecewise linear representation of time series ignores the global characteristics of time series and easily falls into local optima. To solve this, the paper studies the trend in time series and finds its fluctuation. The trends is divided into an upper layer and a lower one with their trend holding points removed. The experimental results show that the segmentation method has low time complexity and is easy to implement, and the fitting error is smaller on the premise of keeping the trend characteristics of time series.