Humic acid salts are produced, with a high output of useful products, by a liquid-phase oxidation of the mixture of an alkaline agent with a lignin-containing plant raw material having the total content of dry substances of 12 20 %, said oxidation being carried out in two stages. In a first stage, pre-oxidation is carried out at a temperature of 50 190 DEG C. and a pressure of 0.5 3 MPa associated with simultaneous treatment with an oxygen-containing gas until a pH of 10.5 12 is reached. The second stage, i.e. oxidation is carried out in the similar conditions at a temperature of 170 200 DEG C. until a pH of 8.5 10 is reached. The by-product of cellulose production by a sulphite process, i.e. concentrated solutions of lignosulphonate or lignin is used as a lignin-containing plant raw material, said lignin being a by-product of hydrolysis production being continuously oxidised by air oxygen in two stages. In order to additionally activate the mixture of the raw material with alkali, operating in cavitation mode, a circulation pump provided with an ejector for stream ejection of air is used. In order to reduce the pre-oxidation time, the first stage can be carried out using peroxide, or by supplying the raw material and alkali directly to a pipeline prior to an ejector and the mixture thereof with an oxidising agent.