Beam axial scanning has been widely applied in the fields of three-dimensional optical imaging and optical material production. However, the existing beam axial scanning methods all have their own drawbacks such as low scanning speed, limited scanning range and damage on the activity of biological samples. Based on focal spot manipulation theory and through performing illumination beam phase encoding, the relative axial position of the focal spot is changed to realize beam axial scanning. The feasibility of the method is verified through simulation calculation. Simulation results show that the scanning method has better linearity and linear fitting factor R2 is 0.993 9. Without mechanical movement, the method has high scanning accuracy, little influence on biological samples, and has potential ability to be well applied in the area of related scientific researches.