The digital image correlation (DIC) method is a non-contact, full-field deformation measurement method. The three-dimensional DIC breaks through the requirement that the surface of the test piece is planar in the two-dimensional DIC system, and obtains the three-dimensional surface displacement field data of the test piece through a stereo or multi-view camera system. First, a local subset is used to fit the least square plane, and the discrete points on the three-dimensional spatial curved surface are projected into the two-dimensional plane. Then, the Savitzky-Golay filter is used to solve the local strain tensor. Finally, the characterization of the strain field of a free-form test piece is realized, especially the errors of the small-plane projection method are investigated. The results show that the calculation error of the strain field caused by the projection process is much smaller than the transmission of the systematic error component contained in the displacement field data. When the strain subset size M=5, the strain field error is about 30 με. When M=10, the strain field error is about 10 με.