The invention relates to a welding method of ultra-thick and high-strength water-power steel, belonging to the welding field. Welding is realized by adopting a mixed welding mode, i.e. hand welding is adopted as a basis for filling the bottom, and gas shielded welding is adopted for a middle layer and a cover part, multilayer and multi-pass welding are adopted, a carbon arc is cropped out after a single side is filled, and the input of welding heat is controlled within 65 kJ/cm. A welding bevel adopts an asymmetric U-shaped bevel, a thick steel plate of 145-155 mm is preheated to 100 DEG C before welding, the inter-layer temperature is kept between 100 DEG C to 160 DEG C, and heat treatment stress is not carried out after welding, and the problem of reheat embrittlement of a welding joint of the high-strength water-power steel is solved. The invention needs simple welding equipment, realizes welding through a combined welding process on the basis of the traditional equipment, reduces the filling amount of welding metal, and achieves the welding effect of a narrow gap. Compared with the hand arc welding, the invention greatly improves the welding efficiency of the thick steel plate and lowers the possibility of welding defects easily generated on the hand arc welding of an ultra-thick steel plate, such as cold cracks, incomplete fusion, crisp fusion, and the like.