Not illegal. It is not illegal to change a graduate thesis into a small thesis, but it must be approved by an undergraduate tutor if it is to be published. Even when published, it depends on whether you choose an undergraduate school or your graduate school as the unit. So, this is still very sensitive. Postgraduate is a kind of academic qualification of higher education and the last and highest stage of academic education in China. According to different classification methods, it can be divided into four types: master's degree and doctor's degree;