The invention relates to a support device, a belt conveyor and a mobile crusher, wherein the support device comprises a first support structure, a second support structure and a third support structure; the first end of the first support structure is hinged to a first base; the second end of the first support structure is used for hinge joint of a rack; the first end of the second support structure is hinged to a second base; the second end of the second support structure is in sliding connection with the rack; the first end of the third support structure is hinged to a third base; the second end of the third support structure is hinged to the first support structure; a power mechanism is arranged on the third support structure; the power mechanism can drive the third support structure to stretch out and draw back to drive the first support structure to rotate to further drive the rack to slide relative to the second support structure and adjust the height and dip angle of the rack. According to the invention, the height and dip angle of the rack can be flexibly adjusted.