The invention relates to a small-particle ternary precursor and a preparation method thereof, the ternary precursor is NixCoyMnzZrk (OH) 2, x is more than or equal to 0.70 and less than 0.98, y is more than 0 and less than 0.30, z is more than 0.01 and less than 0.30, k is more than 0.001 and less than 0.006, and x + y + z + k is equal to 1; and D50 is 3-5 [mu] m. The preparation method comprises the following steps: 1, preparing a mixed solution of Ni, Co, Mn, zircon salt, ethanol, hydrogen peroxide, an additive and urea; preparing a mixed salt solution of Ni, Co, Mn and zircon salt; preparing a sodium hydroxide or potassium hydroxide solution as a precipitator; and preparing an ammonia water solution as a complexing agent; 2, putting the mixed solution into a hydrothermal kettle, carrying out hydrothermal reaction at 120-200 DEG C for 12-18 hours, and carrying out centrifugal washing to obtain an intermediate; 3, adding the intermediate as a seed crystal into a reaction kettle, and adding a precipitator, pure water and a complexing agent to prepare a base solution; wherein the pH value of the base solution is 11.20-11.80, and the temperature is 55-75 DEG C; 4, continuously adding the mixed salt solution, a precipitant and a complexing agent into the reaction kettle at the flow speed of 200-800 mL / min for coprecipitation; and 5, carrying out filter pressing, washing and drying on the co-precipitation product to obtain the internally loose ternary precursor. By adding the zirconium element, the cycle performance is effectively improved. The prepared ternary positive electrode material can relieve volume expansion generated by charging and discharging, and the electrochemical performance is improved.