[Objective] Elucidating the tripping mechanism and the role and status of flower in this process,are of great significant to explor the diverse pollen dispersal mechanism [Methods] The studies were carried out based on the field observation of tripping phenomenon, pollinator visiting behavior, the control experiment of pollen transfer and the anatomical observation of flower structure in Medicago sativa L. cv. Xin Mu No.4. [Results] The specialization structure of wings and keel edge are nested with each other and form the touch sensitive flower structure of alfalfa. The “sensitiveness” structure not only has high selectivity to pollinators, but also can manipulate pollinators" visiting behavior by changing the location of honey hole before and after tripping. Once the keel flap is tripped, about 90% of the pollen in the anther was dispersed, the gynostemium will not return to the original position, and the flower would close on the same day, or it will continue to open 5-6 d, which is a one-time pollen release mechanism. Tripping efficiency of insects, being not related to the insect"s size, determines the tripping contribution rate together with its visiting frequency. The insect"s second visiting did not increase the removing of residual pollen from anthers and pollen deposition on stigma. [Conclusion]The special flower structure and its regulation to pollinators" foraging behavior promoted the occurrence of tripping phenomenon, and benefits maximizing of the male fitness in Medicago sativa L. cv. XinMu No.4.