Abstract: Based on the three-point bending method,an ultra high cycle bending vibration fatigue system is developed and the design method of fatigue test specimens is introduced too.The S-N curve of titanium alloy TC17 is determined by the ultra high cycle dissymmetrical bending fatigue system(20 kHz).The result shows that when stress ratio(R) is-1,the S-N curve of titanium alloy TC17 is a continuously declining curve,The specimens continue to fail over 10⁷ stress cycles.In 10⁷ weeks time,the samples don't happen the fatigue damage,so it does not exist in the traditional sense fatigue limit.