Involuntary admission (IA) is limited to particular situations in mental health laws to protect patients from unnecessary coercion. China's first national mental health law has been in effect since 2013; however, the status of IA has not been sufficiently explored. To explore the changing patterns of IA since the clinical application of the IA criteria specified in the new law, an investigation of IA status was undertaken in 14 periods (each period lasting for one month from 05/2013 to 05/2017) in the tertiary specialized psychiatric hospital in Shanghai. The socio-demographic and clinical characteristics of 3733 patients were collected. The differences among IA rates in different periods were compared, and the characteristics of patients who were and were not involuntarily admitted were analysed. Multiple logistic regression analysis was used to clarify the independent variables of IA. The IA rate dramatically decreased after the implementation of the new law, while the overall trend gradually increased. The implementation of the IA risk criteria is effective, but IA is still common in China. The medical factors influencing IA following the implementation of the new law are similar to those in previous studies at home and abroad. Non-medical factors might be the main causes of the high IA rates in Chinese psychiatric institutions.