Abstract: The specific requirements of a CT-guided minimally invasive surgery robot are analyzed,and a mechanism combining serial structure with parallel structure is put forward.The design preconditions of serial and parallel robot topological structures are discussed.Several structure types of serial robot arms are compared,and then the final structure is determined.For the deficiency that traditional parallel robots have very low dexterity,a structural frame of the parallel mechanism is provided and explained,and the parallel robot structure is ascertained.The characteristic function of the displacement output of parallel mechanism and its degree of freedom are calculated based on screw theory.The characteristic matrix of the displacement output of serial mechanism is also obtained.The simulation with robot dexterity verifies that this hybrid robot can be used to the locating operation in the minimally invasive surgery with limited CT space.This paper provides a theoretical reference for the structure design of new medical robots.