This article introduces a method for measuring the transmittance of an optical fiber bundle. The transmittance of an optical fiber bundle can effectively characterize the light transmittance of an optical fiber, and its transmittance value will directly affect its application in many scientific fields, so it is necessary to accurately measure its transmittance value. The design is based on the solution of the famous Lambert's law of transmittance, and its value is the ratio of the emitted light intensity to the incident light intensity. According to this law, the attenuation coefficient of the fiber bundle can be obtained. The transmittance of a fiber bundle with a certain length is measured by the combination of the truncation method and the reverse order method. The transmittance value obtained from the sample is about 78%, and the attenuation coefficient value is about 0.248m-1. This system is easy to operate, the test results are objective and accurate, and it can complete the measurement of the optical fiber bundle transmittance.