Abstract: Well Shun 6 is an ultra deep well suited in Tarim Basin. The fourth section of 6230-6366m was conducted borehole enlargement to meet construction needs. Structure, working principle and technical features of hydraulic reaming device used were introduced in this paper. Also, operating difficulties were analyzed such as high temperature, high pressure and hard reservoir lithology. To overcome these, measures like small drilling pressure, low-emission cycle and the tool equipped high temperature resistance sealing components were employed. Field test showed smooth borehole enlargement and excellent effect. Borehole diameter was 25 mm larger than that before work which guaranteed the quality of well cementing. The job provided a reference to the application of hydraulic reaming device to ultra deep wells in other blocks.