With the rapid development of e-commerce and the rise of multi-channel retail models， more and more online retailers are adding offline brick-and-mortar showrooms to meet consumers' individual needs with the help of product features and uniqueness displays. Likewise， more and more offline retailers are opening up online channels to increase product sales. When online and offline channels coexist， product purchase channels are gradually diversified， and the "showrooming style" of "offline experience + online purchase" is popular among consumers， but it has also intensified the competition between online and offline retailers. In this paper， the product differentiation strategy of retailer competition under the showrooming phenomenon are studied and the impact of the showrooming and product differentiation strategy on offline and online retailers is explored. The consumer decision process is divided into two stages： the information search stage and the purchase decision stage， each of which has both online and offline channels. First， the learning behavior of consumers in the information search stage are considered， and the self-learning model for offline experience and social learning model for online information search are established. Then， the consumer utility models for different information search-purchase channels are built on this basis， i.e.， the S-S channel， O-O channel and S-O channel， respectively. Based on the Hotelling model and the consumer utility model， the competitive game of offline and online retailers is analyzed. Furthermore， a competitive model of product differentiation of retailers are constructed and the impact of showrooming and product differentiation strategy on offline and online retailers is studied by comparing the base competition model and the product differentiation competition model. The result shows that under certain conditions， the showrooming intensifies the competition between offline and online retailers， while product differentiation strategy alleviates the competition between offline and online retailers to a certain extent and increases the profits of offline retailers. In addition， consumer learning behavior has an impact on retailers， where good Internet Word of Mouth （IWOM） has a positive impact on the profits of online retailers， while the number of online reviews has a positive impact on consumer learning. Some guidances are provided to alleviate the channel conflict between offline and online retailers and promote retail development.