A method and a system for action monitoring of reciprocating sport are provided. The monitoring method is adapted for a monitoring system including a calculation apparatus, at least one gravity sensor disposed on at least one motion part of a human body and at least one electromyography sensor disposed on at least one muscle part of the human body. The method includes: in a process of a reciprocating sport involving multiple actions performed by the human body, sensing a relative angle of each motion part with respect to a reference position of the human body by the gravity sensor and sensing a startup sequence of the muscle part by the electromyography sensor; determining the action performed by the human body according to the relative angle; and determining whether a force applied by the human body is correct according to the action and the startup sequence of the muscle part.