The utilization of high-viscosity paste jetting technology has the potential to significantly expand the range of available materials and enhance the three-dimensional forming efficiency compared to inkjet printing. In this study, the three-dimensional morphology and contour quality of lines printed using high-viscosity silver paste were investigated. Four types of lines were identified based on differences in the printing shape, and contour fluctuation evaluation indices were defined in both the transverse and longitudinal directions to establish quantitative distinction principles. Based on the research of inkjet printing, a modified theoretical model relating the dimensionless line width and droplet spacing was established considering the cross-sectional characteristics of the lines printed by high-viscosity paste jetting. With the establishment of mathematical models, distinction criteria between various line shapes were obtained and the printable range of uniform lines was determined. Finally, based on response surface methodology, the influences of single droplet jetting parameters, the line printing speed, and their interactions on the line contour fluctuation were analyzed. This study involved theoretical and experimental research on the line morphology and contour quality, which can provide support for control of the line printing quality in high-viscosity paste jetting technology.