Objective To study the ultra-high cycle fatigue behaviors of the Ti-6Al-4V alloy implant after subjection to simulated body fluid (SBF) environment exceeding 107 cycles, and to predict its service life. Method The Ti-6Al-4V alloy specimens were subjected to SBF in two groups for two days and six days respectively; using the ultrasonic fatigue testing technique, the ultra high cycle fatigue properties of Ti-6Al-4V alloy after subjection to SBF in body temperature were studied and the initiation mechanisms of fatigue cracks were investigated and analyzed with scanning electron microscopy (SEM) and energy dispersive atomic X-ray (EDA X ray). Then, a comparison with corresponding behaviors of the normal Ti 6Al 4V alloy was made. Results The S~N curve of the Ti-6Al-4V alloy after subjection had the similar tendency as that of the normal one, showing that they descended continuously during 104 and 108 cycles and the descending tendency was lower in the region of 107～108 cycles; the life of the specimens after subjection dropped before the fatigue life of 107 cycles and was smaller after the fatigue life of 107 cycles than that of the normal ones; the life of the specimens subjected for six days had little difference with that subjected for two days; fracture could still occur beyond 107 cycles, showing that there was no fatigue limit as the traditional fatigue conception described. Fatigue cracks mainly initiated from the surface of specimen before the fatigue life of 107 cycles and some specimens had multiple crack initiation sites; fatigue failure initiated from internal inclusion where mostly the element Al aggregated after the fatigue life of 108 cycles. Conclusions The ultra high cycle fatigue properties of the Ti-6Al-4V alloy decreas a little with SBF subjection; the initiation site of the fatigue crack changes from the surface to the internal defect with the increase of the number of cycle.