The study of mathematical problem posing (MPP) from the perspective of a learning trajectory is a new and thriving research field. The latest Chinese Compulsory Mathematics Curriculum Standards stipulate the developmental requirements of students' MPP ability at three different stages: Grades 1-3, Grades 4-6, and Grades 7-9. However, in this study, we found that there were irregular changes in MPP as the grade level advanced for junior middle school students in Grades 7-9. In addition, the results were not the same for different situations. Moreover, it was difficult for students to pose extended mathematical problems. The sample consisted of 904 Chinese students in seventh, eighth, and ninth grades. They each faced the same three tasks involving a real-life situation, a medicine situation, and a mathematics situation. The evaluation of students' MPP ability was based on fluency, flexibility, and profundity evaluation indexes for the three tasks. How to scientifically and reasonably formulate the learning trajectory of MPP is an important research topic for the future.