With the development of e-commerce，online retailers’Double-Eleven Day and other large-scale promotional activities are very popular in recent years． The complaints of out of stock of products and cancelled orders that follow have also attracted increasing attention in recent days． Since price and product availability affect a consumer’s purchase choice jointly，how to manage online retailer’s stock-out level rationally has become a hot topic deserving to be further investigated． This paper firstly studies the online retailers’discount pricing and the shift of market demand during promotions based on consumer choice theory． Then the rationale and evolution path of online retailers’strategic rationing decision are analyzed with consideration of capacity constraint． This paper shows that the ex post strategic rationing outperforms the fully fulfilled strategy when the unit operating cost increases and exceeds a certain threshold point． The ex ante strategic rationing is more profitable than the ex post strategic rationing． With more and more popularity of online purchasing，our study facilitates the understanding of the operations management of internet business and also helps to achieve the healthy development of e-commerce in China.