The invention discloses an illite crystallinity determination method, which comprises: (1) taking a rock sample, crushing, and screening to obtain rock powder; (2) taking the rock powder obtained in the step (1), adding distilled water, carrying out ultrasonic vibration to prepare a clay suspension liquid, and carrying out centrifugal separation and evaporation to obtain a sample with a grade size of less than 2 [mu]m; (3) taking the sample obtained in the step (2), uniformly mixing with distilled water, dropping on a glass slide, and evaporating at a room temperature to prepare an orientation sheet; (4) treating the orientation sheet obtained in the step (3) in 60 DEG C saturated ethylene glycol steam to prepare an EG sheet; and (5) placing the EG sheet prepared in the step (4) into an X-ray diffractometer, scanning to obtain the 10 angstrom diffraction peak of the self-generated illite, and calculating to obtain the full width at half maximum. According to the present invention, the illite crystallinity determination method is suitable for measurement of the crystallinity of various types of clay mineral-containing sedimentary rocks such as mudstone, sandstone, conglomerate, marlite and the like, wherein the test result is stable, the reproducibility is good, various details and the sample treatment condition during the illite extraction and test process are quantitatively determined on the basis of the existing illite crystallinity test, and the error caused by the sample treatment and the manual operation is reduced.