The joint detection probability of orbital angular momentum entangled photons of spontaneous parametric down conversion process was studied. In the case of ignoring the polarization of photon with a thin crystal, expression of the joint detection probability of orbital angular momentum entangled photons was derived. It’s shown that in the spontaneous parametric down conversion process, choice of pump photon, signal photon and idler photon, size of their parameters (waist, orbital angular momentum and radial index) affect the joint detection probability of entangled photons.