The detonation velocity inside the detonator is determined with optical fiber in this paper. It is found that the fluctuation occurs on detonation velocity curve inside the detonator, which is different from the classical description. This discover is valuable for the further study of the detonation development details during the process from ignition to detonation, and is also significant for further improving the structure of the detonator practically. Besides, by comparison of the test results with theoretical values, it proves that applying optical fibers is possible to measure the parameters of detonation.