Formation of uniform pileup morphology and excellent bonding strength between two adjacent metal droplets are essential in droplet-based 3D printing. However, due to the unstable included angle (“gap”) of the pileup droplets generated by the complex coupling effect between the fluid flow and thermal deformation during the printing process, the successive pileup droplets tend to form poor accuracy morphologies, non-uniform properties, and low-quality bonding strength of the formed microstructure. Since the unstable flow and thermal deformation of the liquid metal is inherent properties in droplet-based 3D printing technique with the same parameter conditions, the morphological deterioration or even instability pileup process still remain common challenge in this technique. Here, the effects of velocity and thermal behaviors on the morphology and size of the pileup droplets were revealed. To accomplish this, a droplet pileup numerical model based on the volume of fluid (VOF) was developed to analyze the spreading and solidification mechanism of the pileup process. In addition, a series of printing experiments with the successive pileup of Sn droplets, generated by a self-developed piezoelectric micro-jet device (PMJD), were conducted to demonstrate the effects of impacting velocity and solidified temperature on the pileup morphology. The evolution mechanisms of impacting velocity and heat accumulation effect and its influence on the dimensional accuracy were revealed. Finally, the originality of this work is an ultrasonic-assisted metal droplet pileup (UAMDP) method was first proposed and applied to effectively regulate the pileup morphology and improve the bonding strength. The evolutions of the influence of the velocity field, temperature field, and pileup behaviors during the spreading process were analyzed thoroughly. A surface morphology analysis further showed the effect of ultrasonic vibration on the contact region between two pileup droplets. And experiments showed that such UAMDP (under voltage of VP-P = 15 V) has 5.9 times enhancement in the adhesion tangential force between two pileup metal droplets. This study provides a foundation for further development of metal droplet-based 3D printing technology.