The invention relates to a solar and oil-gas synchronous or alternating and afterheat-applying water heating system and a method thereof. The system comprises a solar heat absorption device, a fuel oil or gas combustor, a combustion chamber, a water heat exchange device, a water storage vessel, a water heat-collecting circulation channel, a gas heat-collecting circulation channel and a switch forcontrolling the water heat-collecting circulation channel and the gas heat-collecting circulation channel to be connected into the solar heat absorption device. For most of the time, the water heat-collecting circulation channel is selected to work, thereby water can be directly heated, and the loss of heat energy is avoided; in frosty seasons, the gas heat-collecting circulation channel is switched by the switch to work, and air is taken as a working medium, thereby avoiding the influence that pipelines are damaged by frost and icing in a water working medium, ensuring normal work of the whole system, and avoiding the resting period. The invention has the advantages of fully utilizing solar energy, having high efficiency and not being influenced by the frosting period.