The present disclosure discloses a three-dimensional topographic mapping system and mapping method. The system includes: a floating platform, a detection apparatus, and a ground apparatus, where the floating platform is located in the stratosphere above a predetermined ground area that is to be mapped; the detection apparatus is arranged on the floating platform and configured to map the predetermined ground area according to mapping instruction information transmitted by the ground apparatus, and return mapping data to the ground apparatus; and the ground apparatus processes the returned mapping data to obtain DEM data. In the three-dimensional topographic mapping system according to embodiments of the present disclosure, the detection apparatus is arranged on the floating platform to map the predetermined ground area and cooperate with the ground apparatus to calculate the mapping data.