Discount is a frequently used marketing tool. This paper is devoted to designing an effective dynamic discount pricing (DDP) strategy in competitive marketing. First, we introduce a competitive word-of-mouth (WOM) propagation model with DDP mechanism. On this basis, we model the original problem as an optimal control problem. Next, we derive the optimality system for the optimal control problem and, thereby, propose the concept of competitive DDP strategy. Finally, through comparative experiments, we find that the profit of a competitive DDP strategy is satisfactory. Our findings contribute to maximizing the marketing profit in the presence of commercial competitors.