Abstract: System for diagnosing pneumatic control valve online include a sensor set having sensors, attached to a pneumatic control valve, for measuring the state of the pneumatic control valve, a smart data analyzer for processing sensor signal acquired in real time from the sensor set and generating pneumatic control valve state information for online automatic diagnosis of the pneumatic control valve, a data server for transmitting, to an online analysis server, the pneumatic control valve state information received in real time from the smart data analyzer and storing the pneumatic control valve state information for predetermined time period, the online diagnostic server for, by comparing the pneumatic control valve state information with pre-stored normal state data of the pneumatic control valve, determining whether the pneumatic control valve is in abnormal state and generating diagnostic information, and a client computer for displaying the diagnostic information received from the online diagnostic server.