Monoclinic fusiform zirconium dioxide (ZrO2) nanoparticles were synthesized by hydrothermal method, a method was established based on monoclinic fusiform nanometer-sized ZrO2 enrichment separation, and trace barium in water was determined by inductively coupled plasma mass spectrometry（ICP-MS）. The detection limit for Ba(Ⅱ) was 0.007 ng·mL-1, and the relative standard deviation was 0.13%（n=11）. The optimal enrichment separation conditions of nanometer-sized ZrO2 for Ba(Ⅱ) were studied in detail, it was found that the percentage of adsorbed Ba(Ⅱ) was more than 99% under pH 10.0 and 2 mL 0.5 mol·L-1 HCl was sufficient for elution of Ba(Ⅱ) by more than 98%. The static adsorption capacity of ZrO2 to Ba(Ⅱ) was 196.6 μg·g-1 and enrichment factor was 250. Properties of nanometer-sized ZrO2 were discussed through regeneration experiment and effects of co-existing ions and contrast experiment to ordinary ZrO2, adsorption properties of nanometer-sized ZrO2 were applied to real samples in the analysis of Ba(Ⅱ) and the determination was carried out by ICP-MS with satisfactory results.