Abstract: The pump adopted by the traditional ballast system, can't fulfill the ballast velocity requirement of man-sized Semi-submersible ships. The compressors have more advantage than the pumps in capacity and size, so large Semi-submersible ships adopt the compressor to complete the ballast and de-ballast. This method shortens the ballast and de-ballast time immensely if the ships sail in the sea, then eventually reduces the ships' work risk and cost. The air ballast system of 50 000 t Semi-submersible ship explained by this article, mainly includes such parts as compressor system, ballast cabin, ballast pipeline system, liquid level measurement system, valve control system and loading computing system. Meanwhile, the ship has completed many international and domestic tasks. The result proves this air ballast system is perfect.