Task-based language teaching (TBLT) has generally been reported to enhance second language (L2) writing on linguistic features such as complexity, accuracy, and fluency. The research on how it influences pragmatic dimensions, however, has been scarce. Drawing upon a pretest-posttest design, this study investigates the potential of TBLT in pragmatic development in Chinese high school students’ English writing. Eighty Chinese senior high school students from two parallel classes participated in this study over four weeks. One class as the experimental group received TBLT writing instruction while the other class as the control group maintained the traditional product approach of writing teaching. Pragmatic outcomes were assessed in terms of functional adequacy (i.e., content, task requirements, comprehensibility, and coherence/cohesion) with functional adequacy rating scales. Results showed the salient effects of TBLT in helping students become more functionally adequate in English writing. In particular, it promoted the content, task requirements, and comprehensibility of students’ writing. The coherence and cohesion performance in writing, however, did not demonstrate considerable improvement. Intriguingly, a significant correlation was identified between students’ English proficiency and the coherence/cohesion dimension in their written texts. The study highlights the effectiveness of TBLT in facilitating L2 writing performance from the perspective of pragmatic dimensions and emphasizes the importance of developing students’ pragmatic writing ability. These findings have significant implications for Chinese high school English writing instruction.