The invention discloses a positioning device for a mandibular ramus osteotomy. The positioning device has two structures: according to the first structure, the positioning device comprises a first main rod, a subordinate rod, a first connector, a positioning rod, a first limiter and an intra-bone nail which takes a fixing effect during an operation; and according to the second structure, the positioning device comprises a second main rod, a guide plate, a first branch rod, a second branch rod, a second limiter, a third limiter, a second connector, a third connector and an intra-bone nail which takes a fixing effect during the operation. When the positioning device is used, an operation incision line can be kept away from mandibular nerve and mandibular nerve tube, and the location of an osteotomy line can be directly and accurately determined; and during the operation, injury of inferior alveolar nerve vessel bundles can be avoided, and a sufficient width with proximal segment can be guaranteed as well.