Location-based services are becoming more and more popular in mobile online so-cial networks(mOSNs) for smart cities, but users' privacy also has aroused wide concern, such as locations, friend sets and other private information. At present, many location sharing protocols in mOSNs have been proposed, but these pro-tocols are inecient and ignore some security risks. In this paper, we propose a new location sharing protocol, which solves these two issues by using symmetric encryption and asymmetric encryption properly. We adopt the following methods to reduce the communication and computation costs: only setting up one loca-tion server; connecting social network server and location server directly instead of through celluar towers; avoiding broadcast encryption. We introduce dummy iden-tities to protect users' identity privacy, and prevent location server from inferring users' activity tracks by updating dummy identities in time. The details of security and performance analysis with the related protocols show that our protocol enjoys two advantages: (1) it's more ecient than the related protocols, which greatly re-duces the computation and communication costs; (2) it satises all security goals, however, most previous protocols only meet some security goals.