A diameter measurement method for a straight pulling single crystal comprises the steps that a CCD (charge coupled device) camera is used for acquiring a single crystal growing image and the single crystal growing image is processed, and the resolution ratio of the CCD camera is higher than 2 million pixels; and the processing of the single crystal growing image comprises the steps as follows: extracting a crystal outline in a position of a single crystal growing solid liquid interface; fitting the crystal outline to obtain an elliptical boundary; correcting the elliptical boundary into a circular boundary; and obtaining the diameter of the single crystal. The diameter measurement method for the straight pulling single crystal is high in accuracy of a measurement result, can be operated easily, and is free from noise disturbance. The method can be used for single crystal diameter measurement during processes such as seeding, diameter enlarging, diameter conversion and equal diameters.