The utility model relates to a locating clamp for machining a main speed reducer assembly blank of a horizontal machining centre. The locating clamp comprises a clamp body connected with a rotating work table of the horizontal machining centre, two speed reducer shell outer circle locating blocks are symmetrically and laterally arranged at the lower portion of the clamp body, and at least two speed reducer outer circle compression mechanisms are arranged at the upper portion of the clamp body. Meanwhile, at least three end face locating blocks are arranged in a circle on the clamp body, and the end faces of the end face locating blocks are respectively on the same plane. A centering and locating assembly for clamping a convex rib is further arranged on one side, corresponding to the convex rib of the speed reducer shell of an automobile, of the clamp body. A bearing hole locating assembly is arranged at the position, corresponding to a main gear guide bearing hole of the speed reducer shell of the automobile, of the clamp body. Two hydraulic control ports are further formed in the lower portion of the clamp body. Compared with a traditional device, the locating clamp improves machining efficiency and device use rate, saves cost and shortens the machining production cycle.