This paper presents an approach for evaluating the usability of portable patient monitors for emergency medical service and proposes a solution for use errors caused by the operation of physical user interfaces and graphical user interfaces. Cognitive load theory is used to analyze use errors in the operation of advanced patient monitors, potentially caused by the limited time in an emergency, as well as user working experience with restricted working memory in critical situations. This study refers to a simplified operation concept and integrates it into a modular interface design (MID) with easy operation that provides sufficient patient physiological information according to user needs. MID for sophisticated medical equipment effectively reduces the user error rate and expands the scope of application to emergency medical technicians in pre-hospital situations, which is critically important with an aging population and a severe shortage of medical professionals. Furthermore, this paper proves that a usability evaluation through cognitive walkthrough is effectiveness in evaluating interface design during the early design stage in the new product development of a medical device.