The positronium time of flight (Ps-TOF) spectroscopy is a novel technique for studying the microstructure in porous materials, in which the high-efficient detection of γ rays generated from ortho-positronium annihilation is the key point for the apparatus setup. In this paper, a new geometry of the scintillator was proposed to improve the γ ray detection efficiency which was indicated by the 22.1% promotion using the simulation results.