Integrating insights from organizational behavior theory and resource management perspective, a moderated serial mediation model was proposed. The mechanisms through which external information acquisition and internal strategic renewal transform unabsorbed slack resources into firm competitiveness were examined. Empirical results derived from a sample of 288 manufacturing firms show that unabsorbed slack resources positively influence firm competitiveness not only through the distinct indirect effects of environmental scanning and strategic renewal, but also through their serial mediating roles. Specifically, competitive intensity negatively moderates the mediating effect of environmental scanning on the relationship between unabsorbed slack resources and competitiveness, while it positively moderates the mediating effect of strategic renewal within the same relationship. Moreover, competitive intensity enhances the serial mediating effect of environmental scanning and strategic renewal. It deepens the understanding of the mechanisms underlying the synergistic effects of different resource management strategies, opens up the black box of the internal mechanisms through which unabsorbed slack impacts firm competitiveness, and provides significant implications for firm managers aiming to devise effective strategies for managing slack resources.