The invention belongs to the technical field of crystallization processes of herbicides and in particular relates to a crystal form C of triazolesulcotrione and a preparation method and application thereof. The X-ray powder diffraction spectrogram of the crystal form C of triazolesulcotrione has characteristic peaks at diffraction angles 2theta of 6.8, 7.6, 9.4, 11.1, 13.1, 13.5, 14.6, 15.5, 16.1, 16.9, 18.0, 19.3, 19.4, 20.9, 21.9, 23.3, 23.6, 23.8, 24.8, 25.5, 26.1, 26.8, 27.8, 28.9, 30.1, 31.3, 33.2, 34.3 and 36.0, wherein the 2theta error range is within +/-0.2 degree. Through tests, the crystal form C of triazolesulcotrione provided by the invention is good in stability; the crystal form is not transformed after being stored for two months at a high temperature of 70 DEG C, the purity is substantially not reduced, and the solubility in water is increased by nearly one time; moreover, and a compound composition of the crystal form C or other active ingredients has a relatively good control effect on gramineous weeds and part of broadleaf weeds in paddy fields, which is obviously superior to that of a crystal form A.