The advent of ChatGPT has surprised many English educators, as theoretically, it has much potential to support English language teaching. However, the empirical results of how ChatGPT influences English as a foreign language (henceforth, EFL) preservice teachers’ teaching plans remain unclear. This study therefore purposed to investigate the effect of ChatGPT on EFL preservice teachers’ teaching plans. This study employed a mixed-method approach and recruited 17 EFL preservice teachers to join the research. The data was collected using a scoring rubric and an interview protocol. Using Mann-Whitney U Test and inductive thematic analysis, this study found that the lesson plans created by EFL preservice teachers in the experimental group failed to outperform those in the control group. Moreover, the pre-lesson plans had no significant difference from the post-lesson ones. The qualitative data explains the statistical results. Then, two implications were drawn to effectively use ChatGPT to support teaching and learning.