To achieve high accuracy of chlorella concentration measurement, using the hollowcore metalcladding waveguide as the measuring chamber for cholrella measurement. Ultrahigh order modes was excited to detect the concentration of chlorella with the free space coupling technique，by monitoring the position and the reflection minimum Rmin of the attenuated total reflection peak of ultrahigh order mode, the concentrations and the absorption coefficients of chlorella solution for the different light wavelength were simultaneously detected. The novel technique is not only achieving detection to 10－9 mol chlorella solution, but also have the advantages of low measuring cost and easy operation, etc.