Recently, a new mechanism of secondary electron multipaction, termed “long-term” multicarrier multipactor, was found in wideband high-power systems used in vacuum environments. Due to the long-term accumulation of secondary electrons between consecutive periods of the multicarrier signal, the long-term multicarrier multipactor has relatively low discharge threshold and is difficult to predict, thus causes potential reliability problems in space and accelerator applications. In this paper, we propose a stochastic approach to the analytical analysis of the multicarrier multipactor discharge occurring in inhomogeneous electric fields. By introducing the random walk and Levy walk theory, the probabilistic model of the lateral diffusion of secondary electrons in the process of a multipactor discharge is derived. Based on the derived probability density, the purely theoretical calculation of the accumulation of secondary electrons of a multicarrier multipactor in a rectangular waveguide supporting TE10 mode is given. The theoretical results comply well with the results achieved by the time-consuming particle simulation, with reducing computational time by about one- order of magnitude. The presented probability density of the lateral diffusion of secondary electrons can have applications in high-power electronics and electromagnetism.