Density functional theory was used to study the effect of N,N-diethylacetamide (DEAc) in the preparation of ferrous oxalate dihydrate. The calculation results indicated that DEAc molecules can affect the growth of ferrous oxalate dihydrate crystals by hydrogen bonding and van der Waals forces with \({\text{C}}_{2} {\text{O}}_{4}^{2 - }\) or H₂O in ferrous oxalate dihydrate crystals. DEAc encases ferrous oxalate dihydrate crystals to inhibit crystal growth, and preferentially binds to both sides of \({\text{C}}_{2} {\text{O}}_{4}^{2 - }\) to control the axial growth of ferrous oxalate dihydrate crystals. The experimental program was conducted based on these theoretical calculation results. Rod-shaped particles with a particle size of 100 nm were obtained by adding DEAc during the preparation of ferrous oxalate dehydrate. Therefore, DEAc had a significant effect on the particle size of the ferrous oxalate dihydrate. The theoretical calculations were thus confirmed by the experimental results. And this work explains the growth mechanism of the ferrous oxalate dihydrate.