What is the ruling on the certificate obtained by a student in China for postgraduate studies, if he faces difficulty writing in Chinese and some exams allow the use of books (open-book exams), and some courses do not require an exam or a report and students pass them automatically?

If there was no fraud, deception, or circumvention of the study regulations and conditions in the way you succeeded, then there is no harm for you in benefiting from the certificate granted to you. The educational institution is the one that assesses and decides your eligibility for the certificate, and it is not necessary for this to be through an examination.