The invention provides a mirror bracket mechanism which combines an unconstraint driving chain with a constraint driven chain, and belongs to the field of a theory of mechanism. The mirror bracket mechanism aims at solving the problems that an existing serial-parallel mirror bracket mechanism is high in coupling, easily causes the mechanism to be jammed, inconveniently conducts motion control and is complex in structure, not beneficial to mechanism design and assembly, high in cost and not beneficial to analysis of the theory of mechanism and modeling. A PPU branched chain module is a constraint driven branched chain module, PSSP branched chain modules are unconstraint driving branched chain modules, each PSSP branched chain module is formed by sequentially connecting a third moving pair, a first ball pair, a second ball pair and a fourth moving pair in series, the fourth moving pairs are driving pairs, the two PSSP branched chain modules and the PPU branched chain module are mutually connected in parallel, the lower end of a moving platform is fixedly connected with one end of the PPU branched chain module, one ends of the two PSSP branched chain modules are respectively and fixedly connected to the moving platform, and one end of the PPU branched chain module and one ends of the PSSP branched chain modules are respectively and fixedly connected to a fixed platform. The mirror bracket mechanism which combines the unconstraint driving chain with the constraint driven chain is used for an optical experiment.