Using density functional theory calculations and molecular dynamics simulations, we systematically explore various possible atomic structures of phosphorene/graphene in-plane heterojunctions and their effects on interfacial thermal conductance (ITC). Unlike the remarkable orientation-dependence of thermal conductivity in pure phosphorene, the ITC is much less orientation-dependent. In addition, the ITC is found to be high, comparable to those of graphene–MoS₂ in-plane heterojunctions and chemically-bonded graphene-metal heterojunctions. Moreover, the ITC of armchair heterojunctions abnormally increases with tensile strain, while the zigzag heterojunctions simply follow the normal trend. To gain an in-depth understanding of these interesting observations, we further analyze the atomic topology and phonon vibrational spectrum and examine the nonlinear interfacial coupling in the heat transport, ITC anisotropy, and temperature effect on the ITC. Our findings suggest that phonon anharmonicity plays a critical role in the thermal transport behavior of two-dimensional in-plane heterojunctions.