Positive emotional design has been shown to have an important impact on multimedia learning. However, before learning multimedia materials, students may inevitably be stressed by external factors such as learning uncertainties brought about by COVID-19, that can affect their final performance. Moreover, whether the principles of emotional design are still valid with the consideration of stress arousal is still unexplored and deserves further investigation. Therefore, this study explored the mixed influence of stress arousal and positive emotional design on learning outcomes (e.g., physiological signals, subjective perception, learning performance measured by test scores). 103 college students were divided into four experimental groups in this study: the SAPE group-stress arousal tasks and positive design (25 students), the SANE group-stress arousal tasks and neutral emotional design (25 students), the NSPE group-not stress arousal and positive emotional design (26 students), and the NSNE group-not stress arousal tasks and neutral emotional design (27 students). Employing an electrocardiogram (ECG) apparatus, questionnaires for data collection, and rigorous statistical techniques for analysis, this research investigates the impact of positive emotional design on students' learning outcomes when subjected to stress arousal. The findings indicated that under conditions of stress arousal, the effect of the positive emotion design on learning outcomes was enhanced. Specifically, analysis of variance was performed on students' physiological signals, regarding the high-frequency (HF) value, and spectral power (SP)value within the [2.2, 2.3] and [3.7, 3.8] ranges, which showed significant interactions between stress and emotional design factors. The spectral power was negatively related to positive emotions. Besides, the analysis suggests that there is a significant interplay between stress and emotional design factors in shaping subjective perception (e.g., learning motivation, experienced satisfaction, and perceived learning). Additionally, compared with the other three groups, the SAPE group performed better on recall and transfer tests. The research results could provide valuable references for the follow-up study of multimedia emotional design.