Abstract: In order to monitor the cement grout density accurately and dynamically in grouting practice, a detection instrument incorporating a CR105 photomultiplier tube as optic sensor was designed. The instrument, which took Cs-137 as light source, CR105 photomultiplier tube as photosensitive device and 196KB single chip processor as micro-processor, realized γ ray detection and data processing through the customized signal process circuit, display circuit and modularized software. The experimental results show that the instrument can successfully monitor the density of the cement grout in harsh environment. The detector of the instrument functions properly when it is installed in the hot, damp and dusty construction site. The measurement is fast and accurate.