This study explores a case where a class of fifth graders’ socioscientific argumentation (SSA) showed unbalanced development in structural quality and justification quality. The students actively engaged in the oral argumentation on “should our city build waste-to-power plants.” Their post-discussion written arguments, when compared to the pre-discussion ones, improved significantly in justification use, multiple perspective-taking, and rebuttals, yet also decrease significantly in the accuracy level of knowledge-based justifications. Tracing the development of the SSA, we identified a few teaching and learning features that shaped this discourse pattern, including an overemphasis on structure, side-taking, context information provided in brief points, and the students’ lack of content and context knowledge. Implications for practice and future research were discussed in reflection.