In elementary science education, it has many potential benefits to engage students in scientific explanations through evaluating claims, evidence and reasonings. This study intends to develop a web-based platform that integrates such functions as science simulation and synchronous online discussion, embedded with argumentation scaffolding, so that elementary students could conduct simulated scientific experiments and construct scientific explanations collaboratively in an interactive online science learning environment. A modified argumentation scaffolding is designed based on Toulmin’s framework and applied in this platform, which consists of six components: claim, evidence, reasoning, agreement, query, and rebuttal. Each component contains several semi-structured or structured text templates to provide guidelines for students’ online practices in argumentation and explanation. An investigation will be made on whether this platform is useful in supporting students’ collaborative construction of scientific explanations in simulated scientific experiments.