Abstract: The equation of soil bore enlarging with water jet was established based on the water jet theory and soil mechanical theory. Through indoor simulation test, the relationship between water jet parameters and bore-enlarging effect was studied. The results showed that the equation of soil bore enlarging with water jet is correct. Increasing the water jet flow quantity has more obvious effect on bore enlarging than increasing the water jet pressure. The experimental parameter β_s is mainly related to the lifting speed and the rotational speed of the drill pipe.