Open modelling problems represent an important type of mathematical problem, but there is a lack of theoretical and empirical research on how teachers judge students’ solutions to such problems. In the present study, 10 pre-service teachers solved four open modelling problems, judged 14 students’ solutions to these problems, and explained their judgements. We applied content analysis to the pre-service teachers’ responses to identify six beliefs they hold about making assumptions when solving open modelling problems. The results indicate that the pre-service teachers’ beliefs might affect their judgements of students’ solutions.