Coding ability has become an essential digital skill for young children. The graphical programming environment is valuable carrier for cultivating children's coding ability. The purpose of this study is to develop a coding ability test for children, to conduct a graphic coding intervention, and to further explore the impact of multiple factors on children's coding ability in ScratchJr intervention. First, the coding ability test for ScratchJr environment was developed based on Item Response Theory to measure kindergarten students' coding ability. The result showed that there are good reliability, content, and structural validity of the test question. Secondly, we conducted a 12-week ScratchJr coding activity teaching experiment on 162 kindergarten students. The analysis of the treated group and the control group showed that coding activity teaching can effectively improve the students' coding ability. Finally, this study analyzed the differences in children's coding ability in coding interest, and coding experience. The results showed that the coding ability was affected by coding experience and coding interest. More interestingly, it was found partial mediation effect among coding ability, coding experience and coding interest. Moreover, ScratchJr coding intervention can influence the direct effect of coding experience on coding ability by improving children's coding interest.