In recent years, swimwear design has become more diverse and more fashionable. Swimwear always plays an important role of casual dress in the fashion industry. However, it has overcome the time and space constraints, and becomes one of fashion's darlings. The swimwear industry offers many exciting fashion job opportunities. What does it take to become an admirable designer? What are core competencies for swimwear designers? This research utilized the competence set theory of habitual domains to examine the core competencies of swimwear designers, Through interviews, questionnaires and data analysis, this research identified the following: 1) the required competence set for swimwear designer; 2) Methods of utilizing designer's specific competence set, and 3) Methods of enriching swimwear designer's existing competence set. Unlike other professions, the swimwear designer must have the skills and expertise that need to be constantly updated and upgraded like a computer to form a great competence set consisting of old and new competence. The designer must keep a creative mode internally by merging all competencies, brainstorming ideas, collaborating and converting work in tandem with great flexibility. Swimwear designers must not only take into account the aesthetics and marketability, they must also put the aesthetic value in the first place and implicitly adhere to objectivity. To become a competent designer, in addition to professional skills, good interpersonal skills, analytical ability and judgment skills, you must also have curiosity, ambition, responsibility, and a proactive lifelong learning attitude.