Promotion can stimulate consumer’s unplanned buying which is regarded as the main source of sales growth, however, it may increase consumer’s planned buying shortfall as well. On one hand, online scarce promotion more significantly influences consumer’s unplanned buying and planned buying shortfall; On the other hand, frequent and continuing online promotion weakens its marginal performance. Under such circumstance, it becomes imperative for us to ascertain whether online scarce promotion can contribute to the growth of consumer’s purchases by comparing the effect of online scarce promotion on planned buying and unplanned buying. Our empirical analysis shows that online scarce promotion can decrease consumer’s planned buying shortfall, and also increase consumer’s unplanned buying, thus adding to consumer’s actual buying relative to their intended purchases.