A method and system for optimal control of ultra-precision cutting. The method for optimal control is based on time-precipitates-temperature characteristics of Al—Mg—Si series aluminum alloy, and includes first determining types of precipitates of machined materials, and establishing a Lifshitz-Slyozov-Wagner (LSW) model of each precipitate. A temperature range is determined corresponding to each precipitate according to the LSW model to obtain a comprehensive temperature range. A relation model is established between cutting parameters and a cutting temperature according to the LSW model. Finally the cutting parameters are optimized according to the comprehensive temperature range and the relation model, so that the cutting temperature is beyond the comprehensive temperature range to inhibit the generation of the precipitates.