I found that most of the articles on the Internet are teaching knitter to divide their yarn by "length" instead of "plies", so I decide to share the way I use with you. If you have better ways to divide yarn, please do not hesitate to share with others.
過程如下 The processes：
Take out the yarn you want to divide by strand
Today's actress - Sable, which is constructed by 3 plies of yarn, so I am going to divide it into 2 plies and 1 ply.
Take off the label. Divide the outer yarn tail in to 2 plies and 1 ply.
Pull out the yarn for about 160cm (it depends, don't be so greedy in the beginning). Use big pin to fix the tail on yarn ball.
Hold the 1 ply yarn by your one hand, and 2 plies by another. Rise your hands and keep the yarn ball in the air, and then it will start to rotate. The 160cm yarn will be divided soon.
Make up the divided part you just done to prevent any trouble afterwards.
Repeat step 3~5, and you will success in dividing the skein of yarn.
Remark: The components of this yarn are 30% Sable fur, 40% Corpilon (?), and 30% warm velvet (?). If you pull the yarn to fast, it is really easy to be broken. This situation is probably happen in every yarn which contains high percentage of fur (or some things like that).
It takes about 20 minutes to divide 40 meters yarn after your are familiar with the steps.