An examination is made of a model of grain-boundary diffusion in which diffusion penetration in the region of the boundary is anisotropic and depends on the distance to the centre of the boundary. In similar to experimental conditions the analytic solutions of the model coincide with the solutions of the Fisher model. The accuracy of the analytic solutions is evaluated by comparing them with the results of numerical solution of the initial equations using the method of finite differences. The model can be used to compare the measured parameters of grain-boundary diffusion and the results of simulation of atomic jumps along grain boundaries.