The invention discloses low-impurity content zinc dialkyl dithiophosphate (ZDDP). In the zinc dialkyl dithiophosphate, phosphorus is volatile impurity phosphorus and available phosphorus; the molar percentage content of the volatile impurity phosphorus measured through a 31P-NMR spectrum does not exceed 7.2% of the total phosphorus content of the zinc dialkyl dithiophosphate. The invention further provides a preparation method of the low-impurity content zinc dialkyl dithiophosphate. The zinc dialkyl dithiophosphate provided by the invention has low impurity content and high available phosphorus content, has excellent volatility resistance and better oxidation resistance, wear resistance, corrosion resistance and other properties, and can effectively reduce the using mount in engine and transmission lubricating oil and reduce the risk of poisoning failure of an exhaust gas treatment catalyst.