Professional cognition is an important part of undergraduate professional training, and also a prerequisite for students to complete professional learning, form professional literacy and become professional talents. By taking H University Library Science as an example, this study collects the first-hand information of students' professional cognition of library science through purposeful sampling and semi-structured interview, comprehensively uses grounded theory and type analysis method to deal with the relevant content, and finds out various factors affecting professional cognition. It is found that curriculum, specialty characteristics, library and teachers are the main factors that affect the professional cognition of the undergraduates of library science. The development process of the professional cognition of the undergraduates of library science includes three stages: prior stage, contact stage and formation stage, and the professional cognition will change in different degrees due to the differences of the stage and influencing factors.