The invention discloses a monitoring device and an online monitoring method of a quick closing process of a steam turbine inlet valve. The monitoring device comprises first to n-th main valve displacement transducers for measuring the opening of a main valve of the steam turbine, first to m-th control valve displacement transducers for measuring the opening of a control valve of the steam turbine, a first pressure sensor for measuring the oil pressure of a servomotor, a second pressure sensor for measuring the oil pressure of AST, a trip circuit for measuring trip signals, a high-speed data acquisition device and a computer. The on-line monitoring method comprises the following steps of initialization, parameter setting, acquisition, alarming, displaying and querying. The monitoring device and the online monitoring method provided by the invention have the beneficial effects of realizing real-time online continuous monitoring of a steam turbine valve closing process, evaluating and early warning the valve state, realizing fast fault positioning, and having an important significance for realizing the predictive maintenance of equipment, reducing the operational security risk, and ensuring the reliable operation of the steam turbine.