The multi-mode parallel mechanism could change the operation mode to meet the requirements of different functional and adaptive tasks. In order to realize the stationary transformation preferably between different modes, an operation mode transformation method of multimode parallel mechanism was proposed. Firstly, to obtain various operation modes and transformation configurations of the mechanism, the dual quaternion was used to describe the position of moving platform systematically, and the reconfigurable analysis of the multimode parallel mechanism was carried out based on the algebraic geometry, that was the polynomial constraint equations were established by using study parameters, and further prime decomposition was performed. Secondly, the reasonable actuation spaces of the operation modes and the transformation configurations of actual demand were analyzed and solved by preferring base pairs or moving pairs close to the base as the driving pairs. The actuation spaces of the mechanism were obtained by intersecting all the actuation spaces of the transformation configurations, and the optimal combination of actuators was selected according to the principles that the least actuators were used to realize the most operation modes and the mechanism had good performance. Finally, TCI (Total constraint index) was adopted to determine the approximate constraint singular space (κ<0.3), and redundancy actuation was adopted in this space to make the mechanism realize the transformation between operation modes smoothly. Taking SNU 3UPU mechanism as an example, seven operation modes and four transformation configurations were obtained, the transformation process between 3T mode and 2T1R mode was analyzed, and the correctness of the method was verified by simulation.