1. A human image detection method for detecting a human image position in a detection area, comprising: receiving by an image processing unit a depth image data from a depth image sensor; removing by the image processing unit a background image of the depth image data for obtaining a foreground image; receiving the foreground image by a human image detection unit, which performs a graph-based segment process on the foreground image to obtain a plurality of graph blocks; determining by the human image detection unit whether a potential human region exists in the graph blocks according to an area size of each graph block; determining by the human image detection unit whether the potential human region is a potential human head region according to a similarity between the potential human region and a hemisphere model if the potential human region exists; determining by the human image detection unit whether the potential human head region is a real human head region according to an area size of a surrounding region adjacent to the potential human head region if the potential human region is the potential human head region; and determining by the human image detection unit the position of the real human head region as the human image position if the potential human head region is the real human head region.