3D reconstruction was adopted to characterize the microstructural morphologies of Mg-3.0Nd-0.3Zn-0.6Zr alloy castings produced by high pressure die casting (HPDC) processes with different parameters, including low slow-shot speed, solidification pressurization and fast slow-shot speed. At low slow-shot speeds of 0.1 m·s-1, 0.2 m·s-1 and 0.3 m·s-1, the porosity is concentrated in the center of the castings with one spiral staggered shape along the liquid flow direction. The porosity volume simultaneously decreases with the reduction of quantity and size of externally solidified crystals (ESCs), while the shrinkage pores become more and more dispersed with the increasing low slow-shot speed. Pressurization not only reduces the porosity volume due to the improvement of feeding ability, but also transformes the center gathered porosity into one layer-by-layer distribution form. Accompanied with the increasing fast slow-shot speed, the central porosity dramatically decreases and transforms into a large-scale spiral staggered shape along the liquid flow direction. However, the porosity is much more dispersed when the speed is increased from 2 m·s-1 to 3 m·s-1.