1. An activity recognition method, comprising steps of: capturing a training video having a first foreground moving object and a first background, wherein the first foreground moving object has a first contour, the steps of capturing the training video comprising: processing the training video to distinguish the first contour from the first background, wherein the first foreground moving object has a plurality of activities; defining a first minimum bounding box for the first contour; calculating a first parameter according to the first minimum bounding box; and transforming the first parameter into a first feature vector; constructing a decision tree model having a plurality of support vector machines (SVMs) for classifying the activities of the first foreground. moving object according to the first parameter and the first feature vector in one of the support vector machines; capturing a testing video having a second foreground moving object and. a second background, wherein the second foreground moving object has a second contour, the steps of capturing the testing video comprising: processing the testing video to distinguish the second contour from the second background; defining a second minimum bounding box of the second contour; calculating a second parameter according to the second minimum bounding box, wherein the second parameter comprises a center value of the second minimum bounding box; providing an algorithm to judge whether the second foreground moving object is the same as the first foreground moving object according to a trajectory in form of the center value varying with the time; and transforming the second parameter into a second feature vector; and each of the support vector machines comparing the first feature vector and the second feature vector in sequence according to the, decision tree model to recognize an activity of the second foreground moving object.