Mathematical model of the process of liquid absorption by a porous-capillary body is proposed in this article. Such model is required in construction materials for theoretical description of the kinetics of liquid absorption by the building materials with porous-capillary structure. It is customary to use ordinary second order differential equation for a one-dimensional description of the motion liquid in a cylindrical capillary (Sheikin A.E., Chekhovsky Yu.V., Brusser M.I., 1979). The replacement of length by volume, it means the transition to the three-dimensional case in this equation, is wrong (Korolev E.V., Grishina A.N., Vdovin M.I., Albakasov A.I., 2016). Attempt to take into account all the physical factors of the medium absorption process by the porous-capillary body is doomed to failure in advance. In such cases it is useful at the same time apply as a probabilistic-statistical approach for analysing the results laboratory experiments, and the apparatus of differential equations for mathematical description of the physical process itself. Differential equation for description the process of liquid absorption by a porous capillary body in the three-dimensional case is taken into account the basic physical laws of the liquid penetration process into a building material with a porous capillary structure and known solutions of the required differential equation. The article presents a mathematical model of the process of liquid absorption by a capillary-porous body; it also studies the time dependence of volume mass absorption by a capillary-porous body, and proposes an analytical expression to account the deceleration process caused by appearance of resistance forces during medium absorption by a capillary-porous body.