Abstract: A method to extend the dynamic range of transmittance meter with a combination of optical attenuator and motor was introduced. In this system, the emission intensity of the LED was controlled by the variation amplitude of modulation signal which was loaded. Furthermore, we selected fixed transmittance attenuator and established the reasonable threshold when the data measurement was carried on. According to fitting analysis of data, the result shows that the system-s dynamic range is extended 0.6 times, when using neutral density attenuator whose transmittance is 40%. And it is extended 6.3 times when the transmittance is 87%. Meanwhile, by choosing neutral density attenuator of different transmittance, the system-s dynamic range can be flexibly extended.