The invention discloses a method for continuously preparing a 2,6-dinonyl naphthalene sulfonic acid. The 2,6-dinonyl naphthalene sulfonic acid is continuously produced through a falling film sulfonation reactor, and the method comprises the specific steps: 2,6-dinonyl naphthalene and a sulfur trioxide gas are continuously introduced into the falling film sulfonation reactor to be subjected to a falling film sulfonation reaction, a sulfonation reaction product is aged, settled to separate out acid residues, extraction and separation are carried out, the separated product is concentrated, acidized, washed with water, evaporated to remove a solvent to obtain the 2,6-dinonyl naphthalene sulfonic acid. According to the method, the efficient production of the high-purity 2,6-dinonyl naphthalene sulfonic acid is realized based on the fact that the 2,6-dinonyl naphthalene is continuously sulfonated in the falling film sulfonation reactor, the process flow is short, the reaction time is short, the facility request is low, the purification of the product is easy, the cost is low, and the industrial production requirements are completely met.