Based on the diffraction theory of the monochromatic scalar wave, an optical trap using a red-detuned small-hole diffracted light field is investigated. The intensity distribution of the optical trap and its optical potential and optical dipole force are colculated by aperture diffraction theory, and derive some analytical relations between the characteristic parameters of the optical trap (including geometric parameters, intensity gradients and theirs curvatures) and the parameters of the optical system (including laser wavelength and aperture radius) are derived. The study shows that when P=500 W, λ=1.080 μm and a=20 μm, the optical trapping potential for cold CH4 molecules is about 57.9 μK. So our trap scheme is not only simple and convenient in the manipulation and control of cold molecular (or cold atoms), but also has wide potential applications in the fields of cold molecular physics, atomic optics, molecule optics even quantum optics,and so on.