Using the sphere-packing theory to generate eight-dimensional modulation formats, we propose a rate-adaptive scheme based on eight-dimensional modulation formats. Compared with the existing rate-adaptive schemes, the proposed scheme can achieve a finer granularity in spectral efficiency only by adaptive switching of modulation alphabet. The simulation results show that the proposed rate-adaptive scheme can adaptively realize the flexible switching of spectral efficiency and the granularity of spectral efficiency is 0.25 bits. Moreover, the bit rate range is 28-84 Gb·s -1 at a fixed symbol rate of 7 GBaud. The proposed rate-adaptive scheme can be applied to the elastic optical network to provide a finer granularity in spectral efficiency and bit rate adjustment.