In this paper, based on the characteristics of rumor spreading in online social networks, we propose a new rumor spreading model. The model considers the impact of changing number of online social network users and user activity. On this basis, we combine the transmission dynamics and population dynamics to set up a rumor spreading SIS model in online social networks with population dynamics. And we numerically simulate the rumor spreading process. From the results of numerical simulation we can know that improved SIS model can be used to characterize the rumor spreading behavior in online social networks. We also give the effective strategies to curb rumor spreading in online social networks.