Triply periodic minimal surfaces (TPMS), a class of periodic implicit surface with zero mean curvature, are emerging as an excellent solution to create graded porous structures. The grading of a porous layer can enhance and broaden the acoustic absorption in a target frequency range. The porosity gradient within the TPMS structure can be precisely controlled by a mathematical function, straightforwardly allowing for optimization towards the desired absorption. As the first step of the optimization procedure, we present a computational approach to determine acoustic absorption properties of functionally graded TPMS porous structures: the transport parameters of a rigid-frame homogeneous TPMS porous absorber are found by the finite element simulations of three static problems, then the absorption coefficient of the TPMS porous structure with graded porosity along the thickness is calculated with the transfer matrix method. The presented approach is validated by direct numerical simulations of the same porous structure.