The invention discloses a liquid heating device which comprises a liquid-liquid heat exchange device, a vapor-liquid heat exchange device and a fryer heating device connected in sequence, wherein the liquid-liquid heat exchange device is a liquid-liquid heat exchanger, and the vapor-liquid heat exchange device is a vapor-liquid heat exchanger. The invention has the advantages that the temperature of the cold liquid can be increased, the temperature of the cooked liquid just coming out of the fryer can be lowered and the wastage of energy can be effectively prevented through the liquid-liquid heat exchange device; the cold-hot exchanged liquid is instantaneously increased to an ideal temperature by the vapor and then enters the fryer through the vapor-liquid heat exchange device, the heating time can be effectively shortened, and energy is saved; and an upper layer, a middle layer and a lower layer of heaters are distributed over the fryer, so that the heating efficiency can be effectively improved, non-uniform heating is prevented, and energy is saved.