The invention discloses a method for preparing sodium lignin sulfonate with high sulfonation degree. A side product, namely wheat straw alkali lignin of a paper-making waste liquid is taken as a main raw material for a free radical degradation reaction under water bath heating conditions under normal pressure at a temperature of between 65 and 75 DEG C, and then an aldehyde substance is dripped to react for 10 to 20 minutes under microwave radiation conditions; and then 50 weight portions of aqueous solution containing 20 to 50 weight portions of sulfonating agent is added into the mixture toreact for 1.0 to 2.5 hours under microwave power of between 300 and 1,200W to prepare the sodium lignin sulfonate with higher sulfonation degree. Compared with the prior alkali lignin sulfonation reaction, the technology prepares the sodium lignin sulfonate with high sulfonation degree and water solubility by initiating the degradation of free radicals of lignin, synergistically coordinating microwave radiation effect, using paraformaldehyde to increase active hydroxyl groups in the lignin, increasing the length of an alkyl chain, and then performing a sulfonation reaction, and the sulfonation degree of the sodium lignin sulfonate is higher than 3mmol/g.