With the aim to measure the biomass concentration in the biodegradation of organic waste gas for a trickling biofilter, an optical fiber Attenuated Total Reflection(ATR) sensor for measuring the biomass concentration was designed and the composition principle, structural parameters, optical path analysis and the theoretical explanation of the sensor were also discussed.A new method to measure the biomass concentration was proposed based on the principles that the evanescent wave is scattered and absorbed by the bacterial suspension when the beams enter into the interface between the optically denser medium-Si crystal and the optically less dense-bacterial suspension, and the receiving bacterial power of the light is changed by the bacterial suspension. Under a temperature of 25℃, the biomass concentration was measured by a visible light source. Experimental results indicate that the mean relative error by proposed method is 2.217%, which can satisfy the on-line measurement requirements of the biomass concentration. The design principle and method of the sensor is significance and can be used in reagent preparation, alcohol production, biochemistry, etc.