The invention relates to the field of energy source recycling, particularly to a seawater source heating and refrigerating combined system used for ships, and preferably to a direct-expansion seawater source heat pump free cooling and heating system used for ships. The direct-expansion seawater source heat pump free cooling and heating system used for ships is characterized in that when low-level cold and heat sources are insufficient, the energy source quality is improved through the heat pump technology to cool and heat the inner spaces of ships; when low-level cold and heat sources are sufficient, a heat pump compressor is bypassed, all throttle valves are opened, a refrigerating fluid circulating pump is opened, the refrigerating fluid performs heat exchange with low-level heat sources through direct expansion so as to realize free cooling and heating of the ships; moreover, a direct-expansion heat exchanger in the system is used for extracting cold and heat energy from low-level cold and heat sources, the shell of the direct-expansion heat exchanger is made from PVC plastics, a coil pipe is a copper pipe coated with an anticorrosive material, intermediate water and tail end water circulation systems in the conventional system are reduced, and the weight of ships selves is reduced; besides, the system has the advantages of energy conservation, environmental protection and good economy, can be easily connected with equipment of ships, and further has energy conservation and environmental protection practical significances.