Abstract: Stellar population synthesis is a main technique to study galaxies and star clusters. I will talk about my works on stellar population synthesis and its application in detail. A main part of my works is the theoretical modeling of stellar populations, which has taken single stars, binaries, rotating stars, and multiple population into account and it can better reproduce many observations. The other part is the application to galaxy and star cluster studies, including the methods and some main results. In particular, I will introduce my new stellar population model and a code for color-magnitude diagram studies.