The purpose of this study is to examine the musical experience of infants in a mixed age group of 1 to 2 years old, which depends on the musical environment surrounding the infant and the qualitative music participation of the teacher. This is a collaborative action study to examine the process of change between infants and teachers. The research question is as follows.

First, what is an effective way to operate for the diverse musical experience of infants?
Second, what about the changes in the infants and teachers according to the rich musical experiences of the children in the grass-leaf class?
1) What are the Changes in Infants?
2) What are the Changes in teachers?

This study was conducted in a mixed class of 1 to 2 year olds in a private daycare center in Seoul for infants. It took about 10 weeks from October 5th to December 11th, 2020. The procedure of the study is based on the spiral self-reflective research model suggested by Kemmis and McTaggart (2000) to identify problems and plan changes, observe the plan''s implementation and change process and results, and reflect on the process and results through this process. The evaluation and revised plans were followed by repetitive action, observation, and reflection. The evaluation method was to analyze the actual conditions and problems of the music environment based on video shooting, photo shooting, teacher consultation, and an objectified test on ‘Infants'' voluntary music behavior'' with a journal through participatory observation, and explored implementation plans.
The results for the purpose of the research are as follows.
First, effective management measures for the diverse musical experience of infants were as follows. In the first run, infants have differences in the level in music due to the teachers'' lack of sensitive interaction, limited musical environment, and mixed-class characteristics. To improve the musical environment, new and household instruments provided new and household instruments. As a result, infants'' behavior and interest in music increased and teachers'' interactions actively changed. Various musical activities were proceeded to provide a more flexible and integrated musical experience for the second practice. As a result, various musical experiences were expanded, but more teacher-led interactions were happened. To improve this situation, the musical experience through play was expanded based on infant interest. As a result, it was able to provide various connected musical experience, cause the effect of providing indirect musical appreciation and make the musical experiences based on infant interest.
Second, the changes between infants and teachers based on the abundant musical experience are as follows. The changes in infants have led to changes in the musical behavior of infants through increased infant-led musical exploration activities, and increased instrumental exploration and free physical expression. With the change of teachers, active musical interactions with infants have increased, and confidence in infants musical education increased as they have enjoyed music with infants.
As the final outcome, teachers need to make an effort to provide individual infants with diverse and rich musical experiences. This kind of diverse musical environmental opportunities should be continuously provided by teachers. So infants have various musical experiences, enjoy and develop their musical level through the diverse and rich musical environment.
In addition, teachers should have a good understanding of how to teach music that can stimulate the interest of infants. To do this, teachers must build confidence in teaching music through teacher education. Moreover, based on this, efforts should be made to enhance practical knowledge of effective music education and the professionalism of teachers.