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.