<p>Mathematics writing (MW) is essential for mathematics learning. To gain a better understanding of MW, we examine the performance of 183 Chinese sixth-grade students on two MW prompts with different levels of mathematical-content complexity: a simple prompt focused on understanding the equal sign and a more complex prompt involving multi-digit operations. The findings revealed that compared to simple MW prompt, students used less mathematics vocabulary, made more grammatical errors, and demonstrated less clarity and precision in complex prompt. Additionally, Chinese students tended to rely more on procedural knowledge, with a decrease in their use of conceptual knowledge as the MW content became more complex. These results suggest that the complexity of mathematical content negatively impacts MW performance. Educators should therefore consider the complexity of prompts when teaching MW to support better student outcomes.</p>