To solve the problem of topological charge measurement in vortex beams multiplexing transmission, a beam propagation model is established based on the extended Huygens-Fresnel diffraction integral, the Fourier optics and split-step beam propagation method. The multiplexing transmission of Laguerre-Gaussian beams in vacuum is numerically simulated. Analysis of the phase distribution after the transmission in vacuum shows that a recognizable association exists between the phase distributions after two beams multiplexing and the topological charge of each multiplexing beam. Specifically, the number and the rotation direction of the central fans are the same as the smaller absolute value of topological charge, while the phase fork of external rim is equal to the difference absolute value of topological charge of each multiplexing beam, and the rotation direction of the external rim is the same as the one with bigger absolute value of topological charge. Therefore, the phase distribution can be used as a new method to measure the topological charge of vortex beams.