The setting method of bollinger bands is relatively simple. Generally, the mid-term moving average is adopted in the middle track, and the default parameter is 20 days. Take the moving average plus 2 times standard deviation for the upper rail, and take the moving average minus 2 times standard deviation for the lower rail. Standard deviation is a quantity to describe investment risk. The bollinger band with twice standard deviation shows that the probability of exchange rate fluctuation in the bollinger band is over 95.44%. The meaning and calculation formula of standard deviation are discussed in the textbook "Love to Death" of all financial planners, and interested investors can refer to it.