1. A motion detection method applied in an interaction system having a processor and an image capturing unit, comprising: capturing a plurality of images by the image capturing unit; recognizing, by the processor, a target object from the captured images; repeatedly calculating, by the processor, a first integral value by integrating a first position offset value of the target object along a first direction over time from the captured images; determining, by the processor, whether the calculated first integral value is larger than a first predetermined threshold value; determining, by the processor, the target object as moving when the calculated first integral value is larger than the first predetermined threshold value; determining, by the processor, whether the target object is moving toward a second direction contrary to the first direction when the calculated first integral value is not larger than the first predetermined threshold value; calculating, by the processor, a second integral value of a second position offset value of the target object along the second direction from the captured images when the target object is moving toward the second direction after a predetermined time period; determining, by the processor, whether the calculated second integral value is larger than a second predetermined threshold value; and determining, by the processor, the target object as moving when the calculated second integral value is larger than the second predetermined threshold value.