In this study, a prompt-based annotation approach is proposed for developing mobile learning systems for architecture design courses. To evaluate the effectiveness of the proposed approach, an experiment was conducted by assigning 56 freshmen randomly to an experimental group and a control group. The students in the experimental group adopted the mobile learning approach with a prompt-based annotation strategy, while those in the control group learned with the conventional in-field instruction and annotations. From the experimental results, it was found that the proposed prompt-based annotation strategy not only promoted the students' self-efficacy, but also improved their learning achievements. In the meantime, it is interesting to find that the experimental group students had medium cognitive load during the field trip, while the control group had rather low cognitive load. This implies that the prompt-based annotation approach engaged the students in mobile learning tasks with reasonable challenges and efforts.