A seawater desalting apparatus using solar energy for continuously supplying heat, the apparatus comprising: a seawater purifying system, and a seawater desalting system; characterized in that the seawater purifying system is a multi-stage purifying system comprising a seawater extraction well for coarse filtration, a seawater sterilizing clarifier, a multi-stage ultra-filter provided with an active carbon filter layer and a multi-fiber filter core layer, and a deoxygenating-decarbonizing tower for deoxygenation and decarbonization arranged in sequence; a purified seawater tank is disposed between the multi-stage ultra-filter and the deoxygenating-decarbonizing tower; the deoxygenating-decarbonizing tower communicates with a purified seawater main outlet; the seawater extraction well communicates with the seawater sterilizing clarifier via a seawater lifting pump; the seawater sterilizing clarifier communicates with the multi-stage ultra-filter via a first seawater transporting pump; the multi-stage ultra-filter communicates with the purified seawater tank via a second seawater transporting pump; the purified seawater tank communicates with the deoxygenating-decarbonizing tower via a third seawater transporting pump; the deoxygenating-decarbonizing tower communicates with the purified seawater main outlet via a purified seawater pump; the seawater sterilizing clarifier is added with a microbicide and a flocculant; and the seawater desalting system comprises: a solar thermal apparatus for concentrating solar energy and collecting heat, a solar energy heat storage tank, a purified seawater heater, and at least one stage of seawater flash evaporator; the seawater flash evaporator comprises: a flash evaporator body and a seawater cooler; a heat transfer medium chamber of the solar thermal apparatus, the solar energy heat storage tank, a first heat exchanger of the purified seawater heater utilizing a heat transfer medium as a heat source and a heat transfer medium pump disposed between the solar energy heat storage tank and the first heat exchanger of the purified seawater heater are connected in sequence to form a sealed route for circulating the heat transfer medium; a purified seawater output main pipe of a purified seawater transporting pump in the seawater purifying system communicates with an inlet of the purified seawater heater and the seawater cooler; a seawater outlet of the purified seawater heater communicates with an inlet of a throttling device of the first-stage flash evaporator body via a heated seawater pump (3 a); the flash evaporator body comprises: a concentrated brine outlet, a freshwater outlet, and a vapor outlet; vapor discharged from the vapor outlet passes through a second heat exchanger of the seawater cooler using vapor as a heat source and is accumulated with a freshwater in a freshwater main pipe; a sealed freshwater storage tank disposed at an end of the freshwater main pipe communicates with a vacuum pump; the concentrated brine outlet communicates with a pipe of a salt plant via a concentrated brine pump; a seawater outlet of the seawater cooler communicates with an inlet main pipe of the purified seawater heater via a condenser body of the flash evaporator body; when at least two stage seawater flash evaporators are employed, the purified seawater output main pipe of the purified seawater pump in the seawater purifying system communicates with the seawater outlets of the condenser bodies of each stage seawater flash evaporator; the concentrated brine outlet of a former stage flash evaporator body communicates with an inlet of the throttling device of a next stage flash evaporator body, and the concentrated brine outlet of a last stage flash evaporator body communicates with the pipe of the salt plant via the concentrated brine pump; and pressures in the flash evaporator bodies of each stage are gradually decreased whereby forming negative pressures.