An approach to measure the tooth root strain of the ring gear in planetary gearboxes using fiber Bragg grating (FBG) is proposed. First, the computing method of the ring gear tooth root strain is studied, and the tooth root strain of the ring gear is simulated. Second, the measurement method is deeply analyzed from three aspects: the strain sensing characteristics of FBG, the installation of FBG, and the structure of the measurement system. Finally, the measurement is carried out. The tooth root strain of the ring gear obtained from the experiment shows obvious single and double meshing sections, meanwhile the range of each section and the strain values at each section are similar with that of the theoretical results, which proves that the tooth root strain of the ring gear can be accurately acquired with the proposed method. Compared with the traditional methods, the new method is more suitable for the real-time measurement of the ring gear tooth root strain in the narrow space of planetary gearboxes.