Rock fragmentation by blasting is the result of the combined action of explosion stress wave and explosion gas pressure. Underground rock blasting is affected by two important factors including the discontinuous surface and in-situ stress, among others. In the discontinuous deformation analysis (DDA) method, the contact damping algorithm is introduced and the search algorithm of blasting chamber is improved. Connected fracture network is searched by a recursive strategy to actualize the real-time tracking of blasting chamber in the blasting simulation of jointed rock. Thereafter, the single-hole blasting of underground layered rock is simulated to study the influence of joint spacing on rock fragmentation under the combined action of explosion stress wave and gas pressure. Results indicate that the attenuation of explosion stress wave in the rock decreases obviously with the increase of joint spacing, and correspondingly the volume of blasting chamber increases and the explosion gas pressure attenuates more slowly. The increase of joint spacing will slow down the attenuation of blasting energy, resulting in more cracks and greater rock fragmentation. With smaller joint spacing, the effect of explosion gas pressure on rock fragmentation is important, and with the increase of joint spacing, the effect of explosion stress wave on rock fragmentation becomes more significant. Moreover, it is found that the radial cracks may deflect along the joint surface and then continue to develop radially or stop. With the increase of joint spacing, more obvious radial main crack development is also observed.