The invention relates to a micro-robot mechanism, concretely relating to a variable-diameter crawler-type robot mechanism used for colon diagnoses and examinations. Therefore, problems of a conventional micro-robot mechanism used for colon diagnoses and examinations are resolved. Such problems include low execution efficiency and the failure of fulfilling functions of varying diameters and lingering. The variable-diameter crawler-type robot mechanism used for colon diagnoses and examinations comprises a variable-diameter sub-mechanism and three walking sub-mechanisms. The variable-diameter sub-mechanism comprises a front baffle board, a back baffle board, a first direct current motor, a driving gear, a lead screw, a driven gear, a front nut, a back nut, three guide rods and three four-barlinkage mechanisms. Each walking sub-mechanism comprises a cabinet, a second direct current motor, a driving bevel wheel, a left bearing, a right bearing, a rotating shaft, a left synchronous pulley,a right synchronous pulley, a driven bevel wheel, a left resistance-decreasing roller, a right resistance-decreasing roller, a left crawler belt, a right crawler belt and an ear board. The variable-diameter crawler-type robot mechanism used for colon diagnoses and examinations is suitable for noninvasive diagnoses and examinations of colon diseases.