A Laguerre-Gaussian (LG) vortex beam is employed as an illumination source for a dark-field microscopy imaging system. To discover the influences of beam characteristics on the imaging quality, an analysis model has been established to show the light-field change rule on both object and image planes. The analytic expressions of the light field on the two planes are deduced. When a rectangular defect is simulated, the light distributions on the object and image planes with different parameters are calculated. The results show that the size of the beam spot on the object plane can be changed by adjusting the topological charge of the vortex beam to obtain the best imaging effect for defects of different scales.