The invention provides a preparation method of a high-temperature-resistant nano wave absorbing agent and a wave absorbing coating. The preparation method mainly comprises the steps that 1, the wave absorbing agent is prepared, specifically, stannic chloride, deionized water, ethyl alcohol and acetylacetone are weighed firstly and then prepared into a solution, inorganic salt such as NaCl is added, a certain amount of solution is evaporated after sufficient dissolution, the solution is placed in microwaves to be heated, and loose powder is obtained; and template materials such as NP-10 are added into the powder, heat treatment is conducted in a muffle furnace after the powder and the template materials are stirred uniformly, and final powder is obtained; and 2, the coating is prepared, specifically, a transition layer containing NiAl is sprayed onto a sand blast alloy plate serving as a base body, and the thickness of the transition layer is 0.05-0.2 mm; the wave absorbing agent is sprayed onto the transition layer, and the thickness is controlled to be 0.5-2 mm; and the prepared coating needs to be subjected to heat treatment for 1-10 h at the temperature of 400-900 DEG C, and finally the high-temperature resistant wave absorbing coating is obtained. The main wave absorbing frequency range of the prepared wave absorbing coating is 8-18-20 GHz, and the wave absorbing performance is reduced by less than 20% at the high temperature of 1000 DEG C.