Laguerre-Gaussian (LG_0l) vortex beam carries optical angular momentum and provides additional multiplexing degrees of freedom, which significantly improve the optical communication capacity. The accurate measurement of the spot size of the LG_0l vortex beam is of great significance owing to its application. The knife-edge method is an ideal method for measuring the spot size and waist radius of the beam for its advantages of low cost, convenience, high precision, and suitability for high-power measurement. However, previous studies on this method focused on the fundamental transverse mode Gaussian beam. Moreover, to the best of our knowledge, there is no research on the measurement of the LG_0l vortex spot using the knife-edge method. In this study, the measurement of the spot size of the LG_0l vortex beam based on the knife-edge method is investigated theoretically and experimentally. In the theoretical analysis, the principle for measuring the LG_0l vortex beam is presented. In the experiment, the LG_0l vortex beam is generated by modulating the fundamental Gaussian beam using spiral phase plate (SPP). The vortex beam spot is measured using the knife-edge method. The spot radii of LG_0l vortex beams (l = 0, 1, and 2) are obtained after fitting the intensity distribution of the transverse field using the Mathematica program. According to the characteristics of the spot radii, we conclude that the spot radius remains unchanged during the transformation from the fundamental Gaussian beam to the LG_0l vortex beam_{using the SPP of lower order.}