Three low viscous phosphate-based ILs [N 1,1,n,H ][DEHP] (n = 8, 10, 12) were synthesized via one step and used for the extraction separation of Nd(III) from aqueous solution under different conditions. The interactions between ILs and La(III), Ce(III), Pr(III), Nd(III) were investigated in depth through DFT method, and the extraction mechanism of Nd(III) using [N 1,1,8,H ][DEHP] was proposed by infrared spectroscopy analysis and slope analysis. [Display omitted] • Three low viscous phosphate-based ILs were synthesized via one step. • Separation factors of Nd/La, Nd/Ce and Nd/Pr are higher than 3 without diluent. • Interactions between ILs and La(III), Ce(III), Pr(III), Nd(III) were studied by DFT. • Extraction mechanism of Nd(III) using ILs was proposed by slope analysis. • Extraction efficiency of Nd(III) using [N 1,1,8,H ][DEHP] is 97% after 8 cycles. Since neodymium (Nd) has similar physicochemical properties to lanthanum (La), cerium (Ce) and praseodymium (Pr), their efficient separation and purification is very difficult. Three new phosphate-based ionic liquids (ILs): N,N-dimethyloctylamine bis(2-ethylhexyl)phosphate ([N 1,1,8,H ][DEHP]), N,N-dimethyldecylamine bis(2-ethylhexyl)phosphate ([N 1,1,10,H ][DEHP]), N,N-dimethyldodecylamine bis(2-ethylhexyl)phosphate ([N 1,1,12,H ][DEHP]) were synthesized and evaluated for selective separation of Nd(III) from aqueous solution. The influences concentration of Nd(III), phase volume ratio (O/A), cation chain length of ILs, extraction time, extraction temperature, salt concentration, solution acidity have on the performance of ILs for extraction separation of Nd(III) from aqueous solution were systematically investigated. It was found that the extraction efficiency (E) of Nd(III) using [N 1,1,8,H ][DEHP] was close to 100% at pH of 4, and that it only required 10 min to reach extraction equilibrium at 303 K. Meanwhile, the separation factors (β) values of β Nd/La , β Nd/Ce , β Nd/pr using [N 1,1,8,H ][DEHP] were all higher than 3. The density functional theory (DFT) results indicated that the interaction between [N 1,1,8,H ][DEHP] and Nd were stronger than that between La, Ce and Pr. In addition, almost 100% of Nd(III) could be recycled from the loaded [N 1,1,8,H ][DEHP] phase using 0.16 mol·L−1 hydrochloric acid via one step, and the E of Nd(III) by regenerated [N 1,1,8,H ][DEHP] remained about 97% after eight cycles. Moreover, the extraction mechanism of Nd(III) using [N 1,1,8,H ][DEHP] was complexation mechanism based on infrared spectroscopy and slope analysis. This work furnishes a strategy for selective separation of Nd(III) using phosphate-based ILs without diluent. [ABSTRACT FROM AUTHOR]