With its ubiquity,the Android system offers more and more functions.One of the important features is the accessibility service,which was introduced in Android 1.6 and optimized in Android 4.0 and above.With the accessibility service,applications can retrieve active window information,such as the text contents and users input,and can communicate with applications' window automatically,for example,getting the button content and then clicking it.However,these advanced features,when being abused,will bring mobile users considerable safety threats.This work carries out research into this system service,analyzing possible ways of being abused and their causes from system API design.The paper then proposes and implements a new system called Tassel,based on the decentralized information flow control mechanism,to make the service API usage securer.The new system tags,tracks and controls the accessibility service handling as well as the accessibility event.This system is evaluated to show that it can prevent accessibility service from being abused,and meanwhile,it does not impact the system normal functions as well as the performance.