The invention discloses a solar energy and mains supply combining water tank heating system which comprises a water tank, a water tank heating pipe, a solar thermal collector, a solar cell panel, a charging device, a power supply selection device, an inverter and a storage battery. The solar thermal collector is installed on the water tank, the water tank heating pipe is installed in the water tank and respectively connected with the inverter and a mains supply through the power supply selection device, the inverter is connected with the storage battery, and the solar cell panel is connected with the storage battery through the charging device. The solar energy and mains supply combining water tank heating system can meet user's water utilization requirements in daily life, can reduce consumption of electric energy of a power grid and can effectively relieve power utilization shortage; power utilization time can be transferred to an off-peak electricity period through the power supply selection device, and accordingly daily power utilization expenditure is reduced; and water in the heat supply water tank can be controlled at a certain temperature through a temperature control device and does not need to be heated again when needing to be used, thereby being convenient and rapid.