A position adjusting mechanism of a soft-closing device for a sliding door has a bracket with a mounting panel and at least one positioning rack, a cover panel mounted on the bracket, and a positioning hook, a torsion spring and a driving element pivotally mounted on the cover panel. At least one first fastener is mounted through the cover panel and the mounting panel and is fastened to a door track. The bracket can slide relative to the cover panel to allow the positioning hook to selectively engage in different engaging recesses of a corresponding positioning rack. Thus, moving distance of the bracket and a door panel suspended from the bracket can be estimated. Accordingly, positions of the bracket and the door panel on the door track can be adjusted to allow the door panel to close a doorway completely.