The invention discloses a method for detecting the diameter of a rotary jet grouting pile. Whether the upper and lower parts of a rotary jet grouting pile to be detected are consistent in diameter is judged by comparing the upper and lower points of the pile in such aspects as sonic time, amplitude of first wave and curve integrity, the detection result is processed to get the diameters at different points, and the obtained diameters of the rotary jet grouting pile are verified by use of a calculation formula. The detection result is more accurate and reliable, and the construction parameters can be changed in a timely manner in the process of construction to make the upper and lower parts of the rotary jet grouting pile consistent in diameter. Through the detection method, quick nondestructive detection of the diameter of a rotary jet grouting pile is realized, money and cost are saved, and deviation can be corrected in time according to the detection result to ensure the engineering quality.