β'' needle-shape precipitate structures significantly affect the mechanical properties of 6xxx aluminum alloys. In this study, a modified multi-phase field (MPF) method combined with the thermodynamic and kinetic databases provided with the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method has been employed to investigate the morphological evolution of the monoclinic β'' precipitates in Al-Mg-Si alloy. Emphasis has been placed on understanding the influences of interfacial energy anisotropy and elastic interaction on the shape of β'' precipitates. It is shown that the high anisotropic strain alone cannot explain the needle shape of β'' precipitates. In the case of considering the contributions from both anisotropic interfacial energy and elastic interaction simultaneously, the simulations lead to the formation of unique micro-needle-like structures in age strengthening 6xxx Al alloy. The simulation results are compared with plate-shaped θ'(Al2Cu) precipitate in Al-Cu system, and verified with high-resolution transmission electron microscopy (HRTEM) images of needle-like β'' precipitates in Al-Mg-Si system.