With the fast growing and globally accepted e-learning technology, Computer-Assisted Learning (CAL) system, which can provide the individualized materials, becomes a matter of great importance. In this paper, we propose a CAL Expert System (CAL-ES), in which teachers can provide their teaching strategy as guidance to students' learning. Because the knowledge hierarchy and the knowledge itself are both important in CAL domain, and the teaching strategy can be considered as rule-format, Two-phase Knowledge Acquisition is proposed to acquire the knowledge hierarchy and the rule-based knowledge by the cooperation of teachers and knowledge engineers. The prototype of CAL-ES is constructed of the mathematical teaching materials. The CAL-ES is useful for student learning and is easy for teachers maintaining.