Due to the rapid development of science and technology, economic development has changed dramatically, resulting in the transformation of career characteristics. Individuals need to convey a higher career adaptability than ever before in order to face the rapid changes brought by development. Especially for college students in the critical period of career development, having good career adaptability is of great significance to their future career choice and development. This study conducted a cross-sectional survey of 692 engineering undergraduates at a top engineering university in China and used the data to investigate the relationship between the professional identity (professional interest, professional strength, career prospects, and professional satisfaction) and career adaptability of college students, as well as to discuss the mediating role of learning engagement in the relationship between professional identity and career adaptability. The results of the correlation analysis showed that professional identity was positively correlated with career adaptability. The mediation effect model indicated that learning engagement played a mediating role in the relationship between the professional identity and career adaptability of Chinese college students. In other words, professional identity had a direct positive impact on career adaptability, while professional identity, mediated by learning engagement, had a positive impact on career adaptability. The study recommends that colleges provide students with a more conducive academic environment and more opportunities for career practice. We also encourage educators to provide more emotional support and identity for students to enhance students’ career adaptability by creating a favorable academic and emotional atmosphere.