Model-based test (MBT) generation techniques for automated GUI testing are of great value for app testing. Existing GUI model-based testing tools may fall into cyclic operations and run out of resources, when applied to apps with industrial complexity and scalability. In this work, we present a multi-agent GUI MBT system named Fastbot. Fastbot performs model construction on the server end. It applies multi-agent collaboration mechanism to speed up the model construction procedure. The proposed approach was applied on more than 20 applications from Bytedance with more than 1500 million monthly active users. Higher code coverage in less testing time is achieved with comparison of three other automated testing tools including Droidbot, Humanoid and Android Monkey.