In view of the problems of existing mechanical bottom expanding drilling tool in rotary drilling construction, such as separation of hole expanding from soil taking and cleaning, complicated processes, difficult sediment cleaning and low construction efficiency, a new type of soil collecting bottom expanding drilling tool is developed. This paper focuses on the design theory of underreamed pile, the structure design of the soil collecting bottom expanding drilling tool, the finite element numerical analysis and optimization improvement of the key components and the prototype trial manufacture as well as field tests. The test results show that this new type of soil collecting bottom expanding drilling tool integrates functions of reaming and soil collecting to reduce the technological processes and improve working efficiency, which meets the construction requirements of rotary drilling for underreamed pile.