The invention provides a preparation method and application of iron ore positive flotation collecting agent, and belongs to the technical field of beneficiation. The preparation method comprises the following steps of (1), mixing illegal cooking oil, methanol and concentrated sulfuric acid according to the mass ratio of the illegal cooking oil, the methanol and the concentrated sulfuric acid as 1.0-1.2:1.1-1.8:0.05-0.09, stirring and reacting to the mixture for 5-12 hours under the temperature of 60-90 DEG C to obtain a product 1; (2) mixing hydroxylamine hydrochloride, water and sodium hydroxide according to the mass ratio of the hydroxylamine hydrochloride, the water and the sodium hydroxide as 1.0-1.3: 64-9.3:1.9-2.4, and stirring and reacting to the mixture for 1-3 hours under the temperature of 20-30 DEG C to obtain a product 2; (3) adding the product 1 into the product 2 according to the mass ratio of the product 1 and the product 2 as 1.0-1.2:0.28-0.48, and stirring and reactingto the mixture for 4-6 hours under the temperature of 40-60 DEG C to obtain the collecting agent. When the collecting agent is used for iron ore positive flotation, the collecting agent can realize the normal-temperature flotation, and is particularly suitable for direct flotation of medium-low-grade iron ores and is low in cost. The production process can fully utilize the illegal cooking oil which threatens the environment and food safety to achieve the dual effects of pollution treatment and comprehensive utilization.