Given the changing nature of literacy, it is critical to investigate how learners communicate using the multimodal affordances of digital platforms. Guided by metafunctions of Systemic Functional Linguistics, this study developed a multimodal analytical framework to quantify multimodal composing with indicators of functional meaning to evaluate students' multimodal literacy practices from an overall perspective. Based on a high school digital literacy SPOC (Small Private Online Course), this study adopted a mixed-method approach to profile students' multimodal posting and examine teachers' and students' perceptions. The empirical findings included that: (1) Five profiles were identified based on students' ideational, interpersonal, and textual meaning differences: academic posters, sensational posters, visual posters, all-round posters, and perfunctory posters. Academic posters viewed multimodal posting as a presentation of research findings; sensational posters viewed multimodal posting as a connection with classmates; visual posters valued the formal beauty of multimodal posts; all-round posters remixed the ideational, interpersonal, and textual functions of multimodal posting; perfunctory posters viewed multimodal posting as a have-to-do task. (2) Teachers underestimated multimodal posting's interpersonal meaning that students valued. The SFL-based multimodal analytical framework and empirical findings provide implications for explicitly evaluating and guiding students' multimodal composition in digital environments.