Recently, two-dimensional (2D) monolayers C3B and C3N attract growing research interest due to the excellent physical properties. In this work, the thermal conductivities (k) of the monolayer C3B x N1-x alloy and the special C3B0.5N0.5 superlattice (C3B0.5N0.5-SL) alloy are systematically evaluated by using molecular dynamic simulation. First, the k of monolayer C3B x N1-x alloy presents a U-shaped profile with the increasing random doping ratio (x), in which the lowest k exists in x = 0.5. Second, we further calculate the thermal conductivity of C3B0.5N0.5-SL. The result shows an initial decreasing and then rising trend, and the coherent length is 5.11 nm which occupies the minimum thermal conductivity. Furthermore, to uncover the phonon thermal transport mechanism, we calculate the spatiotemporal thermal transport, phonon density of states, phonon group velocity, participation ratio and the phonon wave packet simulations in monolayer alloy system. We note that on account of the random doping atoms, the enhancive phonon-impurity scattering and phonon localization reduce the thermal conductivity in monolayer C3B x N1-x alloy. In C3B0.5N0.5-SL, when the period length is smaller than the coherent length, coherent phonon modes emerge because of the phonon interference, in which the superlattice can be regarded as a 'newly generated material'. However, when the period length is larger than the coherent length, the decreasing number of the interface in superlattice lessens phonon-interface scattering and cause the increasing thermal conductivity. This work contributes the fundamental knowledge for thermal management in 2D monolayer C3B x N1-x alloy based nanoelectronics.