According to material mechanics theory and drilling practice, the equation of lateral bending deformation (LBD) of drill was built and the inclined angle of drill tip was solved when drilling small hole. And then the drilling test was done and the character of drilling force was analyzed. Based on this, the LBD of drill was studied. The result shows that, the rise of lateral force and thrust force will increase the inclined angle of drill tip in drilling process, and lateral force is the main factor of generating LBD. By choosing suitable axial vibration parameter, axial vibration drilling can reduce the LBD of drill and improve the dimensional accuracy of hole evidently.