The invention discloses a three-coordinate double-support arm positioner. According to the positioner, when an X-axis movement mechanism, a Y-axis movement mechanism and a Z-axis movement mechanism wholly work, different products at any position in a three-dimensional space can be accurately positioned; a double-support arm structural form is adopted, so that accurate positioning in a narrow space can be realized, and two key points of different types can be positioned at the same time; the X-axis movement mechanism and the Z-axis movement mechanism of the positioner are provided with an absolute grating bar respectively, and full closed loop control is realized by the reading feedback of the absolute grating bars; and a displacement sensor and a mechanical sensor are arranged on a positioning part at the tail end of the positioner, and can be used for monitoring the spatial position and stress condition of a positioning point of a part relative to the positioning part and feeding back an error generated during assembly to a control system for error compensation to effectively support and position the part of an airplane and improve the assembly accuracy.