In previous studies on classroom emotional climate (CEC), factors related to teacher's working conditions have rarely been included. Thus, in the current study, we examined associations between supervisory support, teacher burnout and CEC, and whether pupil teacher ratio (PTR) moderated this association, applying a randomised-control trial design. Participants were students from 300 classes at the end of grade 1, and 300 teachers. Structural equation modelling (SEM) revealed significant relations between supervisory support and teacher burnout and between teacher burnout and CEC. Supervisory support was indirectly related to CEC through teacher burnout. Finally, PTR moderated the association between teacher burnout and CEC.