Abstract: Simple friction oscillator implies complicated stick-slip motion. The premise and basis of stick-slip motion is pure slip motion. For the pure slip motion of friction oscillator with two phases and no viscous damping, it has been discussed by many scholars. As for friction oscillator with viscous damping, its complexity increases greatly. An analytic solution of pure slip motion of the system is presented. The amplitude of the pure slip motion can be expressed implicitly. We can find similar solution structures for different viscous damp ratios. Then a united form of solutions is obtained for different viscous damp ratios. The bounders of pure slip motions are borders of stick-slip motions too, which is so called ssb curves. The analytic expressions of ssb curves are also obtained and discussed, which is important to stick-slip motions. At the end, the stability analysis are presented with the method of Poincare map.