A key aspect in interaction between upwelling mantle plumes and spreading mid-ocean ridges is the along-axis dispersion of the plume, reflecting how far the plume flows along the spreading ridge axis. Observational studies measure the dispersion distance based on the width of geophysical or geochemical anomalies, while theoretical models often define it as the distance reached by plume flow through material advection or thermal diffusion. However, variability in these measurements and the underlying causes remain unresolved. To fill this gap, we explore the dynamics of plume-ridge interactions using three-dimensional non-Newtonian geodynamic models that simulate both material and thermal flow. Unlike previous studies that suggest a steady and uniform ascent, our results show a two-stage plume upwelling process: an initially accelerated ascent from the deep mantle to the mantle dehydration zone, followed by deceleration with lateral dispersion across and along the ridge axis. During the dispersion stage, plume flux and plume-ridge separation distance significantly influence both along-axis dispersion distance and thermal topography of the plume, while ridge spreading rate primarily affects the former. Observations and models consistently show that plume thermal diffusion extends farther along the ridge axis than material advection, with the thermal dispersion distance being approximately 1.55 times greater. We further propose two practical geodynamic indicators—a 0.7 plume material isoconcentration and an isotherm at 0.1 times the excess plume temperature—that can serve as references for estimating plume properties during along-axis dispersion in future studies of more complex plume-ridge systems.