Joint attention skills, broadly speaking, refer to the ability to share experiences and interests about objects and events with others in a social environment. Due to their impairment in joint attention skills, children with autism are usually facing communication challenges, for example, being reluctant to socialize and share items with others. Previous research, aiming to enhance and promote collaboration among children in this population, had designed collaborative games for children to share a single workspace with highly coupled collaborative tasks, which thus could result in forced collaboration and reduce game playability. To address this issue, we design a collaborative puzzle game to indirectly foster players' collaboration and proactive interaction, to train their joint attention skills gradually. The application is different from previous ones in two aspects: (a) It supports loosely coupled collaboration between two players and (b) it focuses on measuring/promoting the joint attention skills among players. Specifically, some puzzle pieces of a player will be systematically put in another player's workspace, and they are expected to notice and share them with the other player to complete the whole game. In addition, the game complexity can also be adaptively adjusted according to the players' playing behavior.