In this paper, a new scheme of generating a three-dimensional array of optical trap is proposed by using a composite phase grating that is fabricated by liquid crystal spatial light modulator. The composite phase grating is formed by combining the circular grating, which is generated by transforming a one-dimensional rectangular grating into a circular grating that can produce the longitudinal array of optical trap, with a two-dimensional rectangular grating. The grating that generates 5× 5× 5 array of optical trap is simulated according to the technical parameters of the spatial light modulator. The output intensity distribution is calculated by using the Gaussian light wave with ordinary power as input light and focusing the diffracting light with lens. The results show that three-dimensional array of optical trap with a very high peak value of intensity and an intensity gradient is obtained around the focus of the lens. The optical dipole potential of trapping cold atoms achieves the order of mK, and the interaction force between the atom and the optical field is much greater than the atom gravity. When the high power laser is used as input light, the generated array of optical trap can also be employed to trap the cold molecules produced by Stark deceleration.