Nowadays, lots of open source communities adopt forum to acquire scattered stakeholders' requirements. But the requirements collection process always suffers from the unformatted description and unfocused discussions. In this paper, we establish a framework ReqForum to define the metamodel of the requirement elicitation forum. Based on it, we propose a lightweight forum-based requirements elicitation process which includes six steps: template-based requirements creation, opinions collection, requirements collection, requirements management, capability identification and the incentive mechanism. According to the proposed process, the prototype SKLSEForum is established by composing the Discuz and its existed pulg-ins. The implementation indicates that the process is feasible and the cost is economic.