Background: The self-learning Methodology in Simulated Environments (Spanish acronym: MAES©, (Murcia, Spain) is a type of self-directed and collaborative training in health sciences. The objective of the present study was to compare the level of competence of postgraduate surgical nursing students in the clinical safety of surgical patients, after training with the MAES© methodology versus traditional theoretical–practical workshops, at different points in time (post-intervention, after three months, six months post-intervention, and at the end of the clinical training period, specifically nine months post-intervention). Methods: We conducted a prospective study with an experimental group of surgical nursing postgraduate students who participated in MAES© high-fidelity simulation sessions, and a control group of postgraduate nursing students who attended traditional theoretical–practical sessions at two universities in Catalonia (Spain). The levels of competence were compared between the two groups and at different time points of the study. Results: The score was higher and statistically significantly different in the experimental group for all the competencies, with a large effect size at every measurement point previously mentioned. Conclusions: The postgraduate nurses were the most competent in the clinical safety of surgical patients when they trained with the MAES© methodology than when they learned through traditional theoretical–practical workshops. The learning of surgical safety competencies was more stable and superior in the experimental group who trained with MAES©, as compared to the control group.