The invention relates to a preparation method of lithium ion battery material lithium iron phosphate. According to the method, tail gas generated in the lithium ion phosphate high-temperature solid-phase method production process is efficiently purified, and a good technical effect and a remarkable economic and environmental benefit are achieved. An ammonia gas purifying tower is used for treating ammonia gas in the tail gas, finally a chemical fertilizer containing ammonium sulfate is obtained and used for sale, ammonium components in the burning process are effectively used, the cost of the mass production is reduced, the environment is protected, and the remarkable economic benefit is achieved. The nitrogen is used for exchanging heat with high-temperature tail gas, the nitrogen is preheated before the sintering, the heat of the tail gas is effectively used, the reaction rate and product purity can be improved, the energy is saved, and the energy consumption is further reduced. Carbon oxide in the tail gas is removed by adopting a carbon oxide adsorption agent, so that harmful gas in the tail gas is further reduced, and the environmental benefit is obvious.