Objectives The purpose of this study is to apply PBL(Problem-Based Learning) to the teaching subject class of preservice early childhood teachers and to analyze how PBL affects the improvement of the key competencies. Methods PBL was conducted for 15 weeks, focusing on making and solving problems that beginner early child-hood teachers may face in the educational field and to preparing a PBL class plan for infants. Participants in the study were 44 second-year early childhood education students who took education methods and educational technology class for the second semester of 2022 at H vocational college in Seoul. In order to analyze whether the PBL class of pre-service early childhood teachers is effective in key competencies, a pre-post survey was con-ducted to diagnose key competencies, and reflection journals and PBL results were collected and analyzed. Results As a result of the pre-post examination, the performance of improving key competencies (creativity, con-vergence, challenge, and growth) through PBL classes showed a significant difference at the statistical .001 level. In particular, the degree of improvement was high in terms of openness to change, comprehensive thinking, goal orientation, and self-efficacy, which are sub-detailed competencies. As a result of analyzing the reflection journal, it was found that students improved throughout key competencies such as creative competency, convergence competency, challenge competency, and growth competency through PBL classes. In particular, as an early child-hood teacher in the future, they showed confidence in the application of PBL classes for infants, and they have expectations that infants, like themselves, will be able to grow through PBL. It also showed a sense of efficacy that various and complex problems that may be raised in the field of early childhood education can be solved coop-eratively through PBL. Conclusions To improve the effectiveness of PBL classes, it is necessary to develop instructors' PBL teaching ca-pabilities, to understand the theoretical and practical instruction design and education methods, make efforts to enhance mutual learning and key competencies through PBL activities, and utilize various teaching methods and multimedia technologies.