While lengthy teacher training programs with reflection were beneficial for promoting teachers' teaching performance and perception of teaching, it is still unclear whether a short online training program can promote preservice teachers' in-depth thinking of their learning and teaching. This study focused on the instructional design of a short self-access online training program with different reflection tasks to stimulate preservice primary and secondary teachers' depth of reflection in China. A qualitative research method was conducted to analyse the reflective logs of 94 preservice teachers. The results showed that developing preservice teachers' depth of reflection during initial teacher education is essential for their professional growth. The results suggested that an online teacher training program with reflection tasks could effectively reinforce preservice teachers' reflective depth by promoting deeper pedagogical knowledge and a stronger professional identity.