The invention discloses a high-precision self-centering clamping mechanism applied for clamping a thin-wall external member. The high-precision self-centering clamping mechanism is mainly composed of a motion rod, conical guide sleeves, a clamping support, an expansion sleeve, compressing steel balls, a spring, a front-end mounting plate, a front-end mounting flange, a rear-end mounting flange, an O-shaped ring, and other parts. According to the clamping mechanism, the compressing steel balls are outwards extruded through the combined action of outer conical surfaces of the conical guide sleeves and a cylindrical surface in the clamping support so as to clamp the thin-wall external member. The processing precision of the compressing steel balls and the conical guide sleeves is high, and the expansion sleeve produces uniform elastic deformation in a radial direction when clamping the thin-wall external member every time, so that the clamping precision in clamping a workpiece every time is ensured; and each compressing ball can universally rotate during clamping, with the design, the mechanism can fulfill a self-centering function during a process of clamping the thin-wall external member, so that the center of the thin-wall external member and the center of the motion rod of the clamping mechanism are kept consistent.