Abstract: In this paper，based on the Lagrange equation of conservative system，the

　　differential equation of three-line pendulum in large swing angle motion and the

　　approximate treatment method of linear vibration with small swing angle are given. Under

　　the approximate condition about the structural dimensions of the three-line pen- dulum，the motion

　　differential equation，period and position probability density distribution of the three-line

　　pen- dulum under two different conditions of large swing angle and small swing angle are discussed.

Key words: three-line pendulum, swing angle, period, position probability density distribution