1. A method for human action recognition, comprising: receiving a plurality of training data, wherein the plurality of training data correspond to a plurality of gestures and a corresponding relationship between the plurality of training data and the plurality of gestures may be one-to-one, or many-to-one; clustering the plurality of training data into at least one group according to a similarity between the plurality of training data; capturing an image sequence of a human action, and obtaining a data representing the human action to be identified from the image sequence; selecting a specific group having a highest similarity with the data to be identified from the at least one group; obtaining a ranking result of all the training data within the specific group through a rank classifier and the data to be identified; obtaining a first training data from the ranking result; identifying the human action as the gesture represented by the first training data, training the rank classifier through a method of learning to rank, wherein the rank classifier reflects a ranking relationship and a data distance of each pair-wise data in all of the training data within the specific group, and the step of training the rank classifier through the method of learning to rank comprises: generating a weak classifier according to all of the training data within the specific group and a weight value of each of the pair-wise data in all of the training data within the specific group; obtaining the ranking relationship of each of the pair-wise data through the weak classifier, wherein the ranking relationship comprises the pair-wise data is concordant in two ranking results of training or the pair-wise data is discordant in two ranking results of training; calculating an accuracy of the weak classifier according to the ranking relationship and the data distance of each of the pair-wise data; updating the weight value of each of the pair-wise data according to the accuracy and the ranking relationship and the data distance of each of the pair-wise data; and repeating each of the steps until a convergence condition is met, and generating the rank classifier through each of the generated weak classifiers and the corresponding accuracy of each thereof, wherein the convergence condition comprises a total number of training rounds reaches a default value or the ranking relationship of each of the pair-wise data no longer changes.