In this paper, we introduce an online video-based system that actively assists users in assembly tasks. The system guides and monitors the assembly process by providing instructions and feedback on possibly erroneous operations, enabling easy and effective guidance in AR/MR applications. The core of our system is an online video-based assembly parsing method that can understand the assembly process, which is known to be extremely hard previously. Our method exploits the availability of the participating parts to significantly alleviate the problem, reducing the recognition task to an identification problem, within a constrained search space. To further constrain the search space, and understand the observed assembly activity, we introduce a tree-based global-inference technique. Our key idea is to incorporate part-interaction rules as powerful constraints which significantly regularize the search space and correctly parse the assembly video at interactive rates. Complex examples demonstrate the effectiveness of our method.