Background Self‐regulated learning (SRL) ability is the key determinant of the success of full‐time online learning. Thus, exploring the influencing factors of SRL and their influencing mechanisms is necessary to improve this ability among K‐12 students. Objectives The purpose of this study was to investigate the influence mechanism of teacher autonomy support on students' online SRL by examining the structural relationship among teacher autonomy support, parental autonomy support, students' self‐efficacy, and students' online SRL. Methods We use structural equation modelling and effect analysis to analyse the collected data from 961 Chinese K‐12 students who engaged in full‐time online learning in their homes during the coronavirus disease 2019 (COVID‐19) outbreak. Results and Conclusions Parental autonomy support and students' self‐efficacy play crucial independent mediating roles in the influence of teacher autonomy support on students' online SRL. Parental autonomy support and students' self‐efficacy have a chain mediating effect on the influence of teacher autonomy support on students' online SRL. Implications On the basis of the results, we suggest that in order to develop students' online SRL ability, it is important for teacher to improve parental autonomy support and students' self‐efficacy. In addition, base on the chain mediating effect, to improve students' online SRL, teacher autonomy support needs focus on parental autonomy support, and then parental autonomy support needs focus on improving students' self‐efficacy. Lay Description What is already known about this topic • Self‐regulated learning (SRL) ability is the key determinant of the success of online learning. • To improve K‐12 students' SRL ability, it is important to explore the influencing factors and their influencing mechanisms on students' online SRL. • Teacher autonomy support play an important role for students' learning. What is not known about this topic? • How does teacher autonomy support influence students' online SRL, especially in full‐time online learning? • Do parental autonomy support and student' self‐efficacy play a chain mediating role in the influence of teacher autonomy support on students' online SRL? What this paper adds • Parental autonomy support and students' self‐efficacy play crucial independent mediating role in the influence of teacher autonomy support on students' online SRL. • Teacher autonomy support has a indirect influence on students' online SRL through the chain mediating effect of parental autonomy support and students' self‐efficacy. Implications for practice and/or policy • Given that parental autonomy support and students' self‐efficacy mediating the influence of teacher autonomy support on students' online SRL, it is important to improve parental autonomy support and students' self‐efficacy. • In order to improve students' online SRL ability, teachers need focus on parental autonomy support, and parental autonomy support focus on improving students' self‐efficacy.