How can game designers realize the balance of “educational function” and “entertainment” in a Computer-based Educational Game (CEG)? The concept of IMS learning design (LD) may be an available method and notion. In this article, a design structure of CEG based on activity theory was set up firstly, and then how to use LD to model each element in CEG was described, which was illustrated by an integration case as well. Finally, the conclusion was got that LD used as a framework of game activity designing can improve the effectiveness of CEG.