Enhancing preservice teachers’ critical reflections on their newly acquired knowledge and experience is crucial for promoting their teaching skills and performance. However, it is a challenging task to teach reflections and increase their depths of reflection. Previous research has succeeded to help preservice teachers reflect in a full-term taught course. However, little empirical research demonstrated the effects of a short self-access online training program on the depths of reflection of preservice teachers. Framed in Ryan and Ryan’s (High Educ Res Dev 32(2):244–257, 2013) reflection depth model, this study adopted a quasi-experimental research design to examine the depths of reflection after attending a short online training of four 30-min sessions varied in training session order and session content. Data of 555 reflective statements were identified subsequently in 120 reflective logs of 30 preservice teachers in a teacher education university in northern China. The results showed a significant difference between the experimental and control groups, indicating that a short self-access online training program has beneficial effects on preservice teachers’ reflections during practicum preparation. While the depths of the reflective statements identified were relatively shallow, the frequency of the reflective statements did not decrease with their depths. Additionally, topics in the online training sessions significantly affected the depths of preservice teachers’ reflections, while the training sequence did not. This study is conducive to designing the relevant online training programmes to promote the depths of reflection of preservice teachers in teacher education programmes.