The idea that information in memory is organized or 'structured' so as to facilitate storage, retrieval, and manipulation is pervasive in modem psychological and educational theory. Therefore, how to properly evaluate learner's knowledge structure and assist their cognitive performance has become an important research topic. Many scholars adopt different methods to measure knowledge structure, the pathfinder method is a objective research instrument. We could obtain pathfinder graph and three indexes of evaluating knowledge structure in using pathfinder method. Pathfinder method offers a potential alternative assessment tool that is not only quantitatively rigorous; but is also consistent with modern theories of knowledge representation. Hence, we use pathfinder method to study senior high school students' physical knowledge structure.