The intensity and phase distributions of a Laguerre-Gaussian vortex beam possessing fractional topological charge are simulated in the far-field, and the evolution of these optical properties is discussed when the topological charge changes from fraction to integer. We find that there are two rings with different sizes and shapes in the intensity distribution maps. The brighter petal-like spots occur on the larger rings. There are ellipse-like dark spots with low-intensity in the circumscribed area of the rings. The number of bright spots and dark spots is related to the topological charge of the incident vortex beam.