The invention provides a semisynthetic aluminum alloy machining fluid and a preparation method thereof. The semisynthetic aluminum alloy machining fluid is prepared from the following components in parts by weight: 30 to 50 parts of base oil, 0 to 12 parts of organic synthesis ester, 7 to 25 parts of emulgator, 3 to 10 parts of corrosion inhibitor, 10 to 15 parts of pH stabilizer, 2 to 4 parts ofantibacterial agent, 0.1 to 0.5 part of defoaming agent, and 10 to 30 parts of deionized water. According to the semisynthetic aluminum alloy machining fluid provided by the invention, the emulgator is prepared from three or more compounds with HLB values ranging from 1 to 15 and with approximate equal-difference gradient distribution, and through the emulgator compound with the characteristics, the semisynthetic aluminum alloy machining fluid has an excellent extreme pressure lubrication performance, high hard water aluminum alloy corrosion inhibition and a high black metal anti-rust propertyat the same time.