Abstract: The instant messaging system is widely used in social softwares. Especially in the mobile internet era, with the diversification of mobile devices, the instant messaging system plays an important role in the mobile platform. Under such background, a cross-platform instant messaging system through the HTTP protocol, XMPP protocol, and SSH, DWR, ExtJS frameworks is proposed. The implementation allows running on multi-platform, including Android, iOS and webpage. A scheme is proposed for the cross-platform instant messaging system design. Finally, a large number of experiments show that the implementation system using the scheme is ideal in terms of reliability and performance.