Abstract: Through a self-developed model test device and PIV technology, the mechanical characteristics of pressured under-reamed anchors under horizontal loads and the development mechanism of the surrounding soil displacement are studied. The test results show that the stress process of the pressured under-reamed anchors can be divided into lateral friction stage, transition stage and end resistance stage, and there will be an "inflection point" in the load-displacement curve between the transitions of each stage. According to the image analysis, it is proposed that the influence range of critical displacement field of soil around the under-reamed anchorage segment of pressured anchor is "bulb-shaped". When the length-diameter ratio of the enlarged head anchoring section exceeds 3∶1, the displacement field at the proximal end of the under-reamed anchoring section is basically unaffected with the continuous increase of length-diameter ratio.