When applying for a doctoral degree, it is usually required that the applicant's master's thesis is consistent with or closely related to the doctoral thesis field. This is because doctoral research needs to further explore and expand research in this field on the basis of master's research. If the research field of master's and doctoral dissertations is too large, it may lead to difficulties in the research foundation and research methods, and it is difficult to successfully complete the research tasks at the doctoral level.

However, if the applicant has accumulated rich research experience and achievements in the master's stage, and has a strong interest and research motivation in another field, and has been recognized by the tutor, then he can also consider turning to another field of research in the doctoral stage. In this case, the applicant needs to fully demonstrate his research ability and understanding and exploration of the selected field to prove his ability to complete interdisciplinary research tasks.

In short, the doctoral thesis should be consistent with or closely related to the research field of the master thesis, but the specific situation needs to consider the applicant's research experience, interests, research motivation and the opinions of the tutor.