The present invention discloses a pollution-free hydraulic oil used for high-precision hydraulic part and a preparing method thereof. The pollution-free hydraulic oil comprises the following components: 30-65% by mass of saturated alkane with 12-14 carbon atoms, 10-35% by mass of synthetic ester composed of higher fatty acid and polyhydric alcohol with 12-14 carbon atoms, 10-35% by mass of high molecular polymer, 0.05-2% by mass of sulfur-phosphor-zinc salt, and 0.05-1.5% by mass of amine salt. The preparing method of the invention comprises the steps of: firstly preparing the synthetic ester;then after mixing the synthetic ester with the saturated alkane with 12-14 carbon atoms, slowly adding the high molecular polymer, the sulfur-phosphor-zinc salt and the amine salt compound. The hydraulic oil and the rolling technical oil provided by the invention have excellent intersolubility. Not only can the lubricating property be guaranteed, but also the cleaning property is excellent as thecarbon number is low.