To measure the center of gravity of a large-sized aircraft, a flexible center of gravity measurement method is proposed. A method to obtain the line that passes through the center of gravity, the coordinate transformation, and the means to obtain the center of gravity itself are also proposed. First, the projection of the line that passes through the center of gravity is measured, and a line that passes through this projection point is established and converted into a coordinate system of the body under test. Second, by means of rotating or tilting the body at any angle, the second line using the same method is measured and converted into the coordinate system of the body. The center of gravity is obtained by finding the intersection of the two lines. Finally, a Monte Carlo simulation analysis method is used to determine the method's uncertainty. Both the simulation analysis and experimental results showed that the center of gravity measurement error is within 0.3 mm. Thus, the proposed method can achieve high-precision detection of the center of gravity even when the position between the device under test and the measuring device is unknown.