Constructivist Learning Theory (CLT) has been a major theoretic base for language teaching reform. The goal of computer-assisted instruction (CAI) is to use computer to partly replace teachers’ complex mental labor which can be done only by the improvement of the intelligence of CAI. Therefore, Intelligent Computer Assisted Instruction System (ICAI)emerged, which brings artificial intelligence technology into the CAI system, and gives computers advanced intelligent systems as human’s. Theoretical analyses and empirical researches of teaching software in ICAI prove the importance of teaching software design in CLT-oriented, ICAI-based English teaching.