There is widespread agreement that science learning always builds upon students' existing ideas and that science teachers should possess knowledge of learners. This study aims at investigating pre-service science teachers' knowledge of student misconceptions and difficulties, a crucial component of PCK, on Newton's Third Law. A questionnaire was designed and conducted to 143 pre-service science teachers enrolled in a normal university in China. A comparison between participants'predictions and student actual outcomes was detected. The result revealed a tendency for pre-service science teachers to under-predict the problem-solving ability of senior high school students. Furthermore, most pre-service science teachers neglected two common learning difficulties in Newton's Third Law. It seems that there is a need to help pre-service science teachers be aware of their own misunderstandings about students.