Informed by a sociocultural perspective on second language teacher education, the present qualitative study investigates three preservice teachers' (PSTs) writing instruction during the English as an Additional Language (EAL) practicum in Australian secondary schools in relation to the multidimensional context of the practicum and the PSTs' personal backgrounds. Sources of data included individual interviews with the PSTs and their school mentors, lesson plans and recordings, teaching materials, the PSTs' self-reflections, and analysis of the schools' EAL programs. Data analysis revealed that the main factors shaping PSTs' writing instruction included the EAL programs at the schools, school teachers and the mentors support, EAL students' background, proficiency levels and responses to tasks, and the PSTs' knowledge, prior education and work experiences. Based on the findings, the paper discusses implications for enhancing the quality of EAL teacher education, especially for promoting productive coursework and placement experiences for PSTs.