A preparation method of battery grade ferrous oxalate for lithium iron phosphate includes the following process flow:

1. Add pure water into the decluttering vessel, start stirring, and add ferrous salt solid according to the liquid-solid weight ratio of 0.5-5:1 according to the amount of pure water, and start heating. After the water temperature rises to 80-100 C, add reduced iron powder with the weight ratio of ferrous salt solid 0.5-3%, stir at constant temperature for 1-3 hours, control the reaction pH=2-4, get ferrous salt solution, and start pressure filtration.

2. The ferrous salt solution produced in step 1 is filtered through a set of auto-pressurized sealed chamber press filters, at the same time, metal impurities, non-metallic impurities and excessive reduction iron powder are removed to obtain pure ferrous salt solution.

3. When the filtrate of ferrous salt in step 2 is clear, open the outlet valve of the filter press and quickly pour the filtrate of ferrous salt into the main reactor; when the filter press is completed, the temperature is controlled at 30-60 C, and the molar number of ferrous salt is 1.001-1.05 times of that of dihydrate oxalic acid solid, the feeding time is 5-60 minutes, and the reaction temperature is raised to 70-60 minutes after 10-60 minutes. Constant temperature reaction at 100 C;

4. Ferrous oxalate slurry was obtained by fully stirring the reaction liquid obtained in step 3 and aging at 2-4 H.

5. Separation of ferrous oxalate from ferrous oxalate slurry by centrifuge at a speed of 30-100 rpm and a centrifugal time of 20-60 min;

6. The separated ferrous oxalate was washed and dried to obtain the finished ferrous oxalate.