In recent years, growing interest is shown in Technology-aided formative assessment (TAFA) and language learning. Research has shed light on the experimentation and effectiveness of various TAFA tools, focusing on their pedagogical advantages in assisting the teaching of particular linguistic skills (reading, writing, spelling, etc.). Taking an ecological perspective, this paper reports on an ethnographic case study on the various affordances perceived by a group of English as a Foreign Language (EFL) teachers who used a learning management system "designed for Formative assessment (FA) purposes" in China. Data from interviews, teacher journals, and EFL classrooms are collected. Thematic coding of interview data shows that the platform offers a wide range of pedagogical, managerial, assessment, social, and developmental affordances for the EFL teachers, with experienced teachers in FA reporting better affordances both in quality and in quantity. This paper further proposes a model to understand how TAFA can be designed in a way that fosters EFL teaching and student learning. Implications for designing TAFA and future studies are also discussed.