1. A method for detecting an object, comprising: obtaining a plurality of depth images of the object by picking up the object by a plurality of stereo cameras; extracting from the depth images, foregrounds with an image coordinate system; fusing the foregrounds in a unified three-dimensional world coordinate system defined by rectangular coordinates of x, y and z directions, where the object stands on an x-z plane limited by x and z axes, to generate a foreground fused in the unified three-dimensional world coordinate system; calculating an appearance two-dimensional histogram on the x-z plane of the foreground fused in the three-dimensional world coordinate system by dividing the foreground fused in the three-dimensional world coordinate system into vertical members extending along the y direction and corresponding to objects on the x-z plane, and getting statistics of numbers of foreground points associated with at least one of the vertical members so as to obtain the appearance two-dimensional histogram representing the numbers of the foreground points associated with at least one of the vertical members; for at least one of the vertical members, determining whether a detection position of the vertical member within the foreground fused in the three-dimensional world coordinate system is within an overlapping region of fields of view of the stereo cameras and determining a number of stereo cameras capturing the overlapping region, based on placements of the stereo cameras; determining a detection parameter relating to the detection position of the at least one of the vertical members, wherein the determined detection parameter is an object template corresponding to an object standard surface area at the detection position, the object standard surface area being a function of an overlap number and a region where none of overlaps exists; and detecting the object as the at least one of the vertical members, based on a comparison operation using the determined detection parameter.