Instant communication applications in Mobile Internet have become an indispensable part for people's daily communication nowadays. The real-time communication protocol, which is the basis for the development of instant communication applications, however, has no unified standard. The existing real-time communication protocols are not able to perfectly adapt to mobile Internet, which has a constrained network environment with the characteristics of low-bandwidth, limited processing capabilities and high latency. MQTT is a publish/subscribe based, extremely simple and lightweight messaging protocol, whose design principle is to minimize the network bandwidth and device resource requirements. This article first introduces some existing mainstream instant communication protocols and points out their shortcomings in mobile Internet environment. The second part introduces MQTT protocol briefly and studies the format and interaction process of MQTT message. Finally, based on MQTT, the article puts forward an implementation of IM and Presence, which are the core functions of Instant Communication. As a conclusion, the experiment shows that using the MQTT protocol can provide less bandwidth cost and better user experience for instant communication in mobile Internet.