Abstract: This paper is to find out the strain state, variation and development of cracking in unsaturated expansive soil during dry-wet cycle. A model box with 150cm long, 10cm wide and 60cm high is designed. A high spatial resolution distributed optical fiber strain measuring technique(PPP - BOTDA) is used to monitor the strain status of the sensing fibers embedded in unsaturated soil at 4 different depths during the dry-wet cycle. At the same depth, the sensing fibers include 4 different kinds and are laid along the longitudinal direction of the model. The experiments show that strain of the sensing fiber is under the influence of soil cracking. Spatially, the strain status is not uniform and characterizing with alternate distribution of tension and compression. Temporally, with the decreasing of soil water content, the stain state of the sensing fibers across cracking zone changes from compression to tension and the stain value has correlation with the total crack opening. With the increasing of the embedded depth, strain distributions of the sensing fibers are more uniform as the influence of the surface cracks to the deep fibers is relatively small. During the wetting process, the strain state of the sensing fibers across the cracks changes from tension to compression with the closing of cracks, whereas the fiber strain near the cracks goes tensile. The results indicate that PPP-BOTDA based distributed optical fiber sensing technology can monitor the strain state of expansive soil during dry-wet cycle and locate the cracks of the soil accurately. The PPP-BOTDA can be a new technical means for soil deformation monitoring and disaster assessment.