The invention provides a preparation method and application of a phosphorite forward-flotation collecting agent and belongs to the technical field of ore dressing. The preparation method comprises thefollowing steps that (1) illegal cooking oil, methanol and concentrated sulfuric acid are mixed according to the mass ratio of the illegal cooking oil to the methanol to the concentrated sulfuric acid being (1.0-1.2): (1.1-1.8): (0.05-0.09) and react under stirring at the temperature of 60-90 DEG C for 5-12 h, and a product 1 is obtained; (2) hydroxylamine hydrochloride, water and sodium hydroxide are mixed according to the mass ratio of the hydroxylamine hydrochloride to the water to the sodium hydroxide being (1.0-1.3): (6.4-9.3): (1.9-2.4) and react under stirring at the temperature of 20-30 DEG C for 1-3 h, and a product 2 is obtained; and (3) the product 1 is added into the product 2 with the mass ratio of the product 1 to the product 2 being (1.0-1.2): (0.28-0.48), the product 1 andthe product 2 react under stirring at the temperature of 40-60 DEG C for 4-6 h, and thus the collecting agent is obtained. When used for forward-flotation of phosphorite, the collecting agent prepared through the preparation method is high in selectivity, high in ore carrying capability and capable of achieving constant-temperature flotation.