Multi-walled carbon nanotube (MCNT)-enforced cross-linked polyimide aerogels (MPIAs) show wide design scenarios and engineering applications due to their excellent physical and chemical properties, such as high flexibility, efficient heat insulation, ultra-low density, thermal stability, and strong mechanical properties. A comprehensive micromechanism-based inelastic constitutive model is developed for MPIAs under finite deformation framework in this work, aiming to provide a theoretical foundation for their designs and engineering applications. The overall thermomechanical properties of MPIAs compose of two superimposed chain networks of the physically cross-linked polyimide aerogel network between nanotubes and the cross-linked chain network of polyimide aerogels. Considering uniformly distributed chain lengths of each nanotube pair, a total energy potential is integrated by applying classical microsphere full network model to capture mechanisms of nanotubes interacting with polyimide aerogel networks, where a continuous damage kinetic model is developed to characterize gradual detachments of chain networks. The significantly enhanced material modulus and toughness of MPIAs are captured by applying two flexible beams to model the two-end connected chains and dangling chains. An equivalent viscoplastic deformation mechanism is applied to model the pores breaking, the porosity variation as well as the chains slipping, which are found to affect greatly the material toughening. The linear elastic behavior of cross-linked neat polyimide aerogel chemical network is considered, and the overall energy function density is integrated for MPIA by combining two superimposed networks. The overall model is calibrated by a series of experiments and then applied for simulations of isothermal uniaxial tension of MPIAs. The underlying mechanisms are further discussed for significantly enhanced modulus and toughness of MPIAs by adding different contents of MCNTs. This work provides theoretical understandings and design guidelines for MCNTs-enforced polyimide aerogel.