Mobile Office Automation is a popular application on the mobile platform. However, the mobile platform has no enough security protection in front of the open system, such as internet. The document security problem in this application has become a hot topic. This paper proposed a new solution to this problem. The solution based on the trusted computing technology, which implements the platform security by hardware. The solution also includes the transparent encryption technology that means application could run independent to the security module and keep the consistency of user experience. The trusted mobile client platform architecture and security functions process of the mobile document protection system are detailed. Related work also been introduced; the security analysis shows the proposed solution could provide well security enhancement for document protection in mobile platform.