A curved surface auxiliary measuring device and a curved surface coordinate system determining method for symmetrical double-curvature panels relate to the field of aviation and aerospace manufacturing machinery. The device and the method are provided to solve the problem that the existing curved surface auxiliary measuring device and the coordinate system determining method are complex and need large amount of calculation. According to the technical scheme, the upper and lower end faces of a base are kept parallel to each other, two intersecting side walls on the base are respectively equipped with a first fixed seat, the remaining two side walls on the base are respectively equipped with a boss, each boss is provided with two internal threaded holes, and one end of each bolt is fixedly connected to a second fixed seat after passing through the corresponding internal threaded hole. The method comprises the following steps: step 1, putting and fixing a curved piece in the device; step 2, acquiring point cloud data; and step 3, establishing a coordinate system, and using Imageware software to process the point cloud data. The curved surface auxiliary measuring device and the curved surface coordinate system determining method of the invention are used in auxiliary measurement of the curved surface of a symmetric double-curvature panel.