The utility model relates to a tensile force adjustment mechanism and have its flexible drive assembly. The tensile force adjustment mechanism include: elastomeric element for make the tensioning of flexible gear spare, an adapting unit and the 2nd adapting unit, one is fixed for the action wheel, and another is used for connecting elastomeric element, an adapting unit includes along a plurality of first location portion that the first direction was arranged, the 2nd adapting unit includes along a plurality of second location portions that the first direction was arranged, an adapting unit and the 2nd adapting unit can follow first direction relative motion, with the elastic force of adjusting elastomeric element, every first location portion homoenergetic is for any second location portion location, so that the 2nd adapting unit is fixed for an adapting unit, interval between two adjacent first location portions is L1, interval between two adjacent second location portions is L2, wherein, L2>, L1, and L2 is not equal to nL1, n is the natural number. The lower problem of regulation precision of the tensile force of flexible gear spare has been improved.