MAX-phase compounds with layered structure have been investigated extensively due to their excellent metallic and ceramic properties. Among them, Cr2GaC and Cr2GaN possess the same crystal structure and similar electronic structure, whereas Cr2GaN undergoes a spin-density-wave transition around 170 K but Cr2GaC is Pauli paramagnetism without any magnetic transition. To understand what make these happen, we studied the crystal structure between 300 and 35 K for both Cr2GaC and Cr2GaN. As the temperature decreases, the lattice constants (both a and c) decline monotonically in Cr2GaC, while an abnormal negative thermal expansion of c axis was observed below 170 K in Cr2GaN. The increase of c/a induced by abnormal increase of c leads to the enhancement of localization of Cr2N plane and causes more Cr-3d electrons confinement in Cr2N plane of Cr2GaN. This further facilitates the reconstruction of density of states near the Fermi surface and even forms the Fermi surface nesting, resulting in the spin-density-wave transition as well as the abrupt changes of resistivity, specific heat, and density of carriers around 170 K in Cr2GaN.