In this paper, the sine-squared potential is introduced, and the motion equation of particle is reduced to a pendulum equation. with constant moment; the propeties of the phase plane are described in detail by using the method in nonlinear mechanics, the effects of the singular point distribution and motion in phase plane on stability of the system are discussed, the critical parameters and maximum energy are deduced. Besides, the particle motion behaviours in the case of small amplitude approximation is analyzed. The results show that the system appears to exhibit soft-spring properties. The solution and the period are represented exaclly by Jacobian elliptic function and the 1-st kind of complete ellrptic integral. The space distribution and momentum distribution of particles are derived.