HarmonyOS is a new all-scenario operating system. As its software ecosystem rapidly expands, how to conduct automated testing of HarmonyOS apps to ensure app quality has become a crucial task. Model-based testing has been shown to be an effective method for automatic Android app GUI testing. Inspired by previous work, we in this work explore how to perform model-based testing for HarmonyOS apps. To characterize app behaviors, we first propose the page transition graph model, which is a directed graph describing transitions between various UI pages in a HarmonyOS app. We then devise a static analysis method to build page transition graphs from the source code of HarmonyOS apps. Leveraging the model, we implement a testing tool which can effectively perform systematic GUI exploration in HarmonyOS apps. We have evaluated our tool using 10 popular open-source HarmonyOS apps from GitHub and Gitee. Experimental results show that the extracted models are highly precise. Moreover, within the same time budget, model-based testing significantly improves the test coverage of HarmonyOS apps over a random baseline method. Our tool is open-sourced at https://github.com/sqlab-sustech/HarmonyOS-App-Test and a video demo is at https://youtu.be/dgZWkHiBYbA.