Considering the characteristics of college teachers job, an extended form of Cobb-Douglas production function was built to reflect the outcome of teachers job in the circumstances of multi-task efforts invested in teaching, research and administrative affairs. A multi-task job engagement incentive model for teachers was established based on the principal-agent theory in order to analyze the influencing factors and mechanism of multi-task job engagement for teacher. Results indicate that the optimal level of job engagement to each of the three tasks is positively associated with the comprehensive balance skill level, and is determined by the relative importance to teachers job engagement and the relative input cost coefficient of each task. Besides, the optimal incentive coefficient for teachers is influenced by two kinds of factors: one kind is related to the teachers, including the comprehensive balance skill level, absolute risk-aversion coefficient and unit input cost coefficient, the other kind is related to the significance of each task and the uncertainty of the external interference factor. Accordingly, puts forward some policy suggestions on motivating teachers job engagement under the multi-task balance from building a comprehensive balance skills upgrading mechanism, establishing contingency incentive mechanism and designing multitasking balance performance evaluation mechanism.