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Single-point diamond turning is a high-efficiency, low-cost method for manufacturing harmonic diffractive optical elements (HDOEs). Generally, in order to ensure high diffraction efficiency of HDOEs, a half-round tool is used to reduce surface-relief profile errors and a super small feed rate is selected to control surface roughness when hard-brittle materials are turning. However, this method is no longer suitable for soft-brittle materials, which have a strict requirement for the range of the feed rate. It limits the types of materials available for HDOEs. Therefore, according to the range of the feed rate and the cutting depth for soft-brittle materials, an optimized turning model is proposed in this paper. It overcomes the high surface roughness caused by the half-round tool; meanwhile, the advantage of the half-round tool is kept in terms of surface-relief profile errors. On this basis, a mathematical model is proposed to reveal the relationship among diffraction efficiency, period widths, tool radius, and feed rate with different soft-brittle optical materials. As a typical soft-brittle material, barium fluoride ( B a F 2 ) was selected to be the material for manufacturing HDOEs. By using the optimized model, the turning experiment of B a F 2 HDOEs was completed, and then the B a F 2 HDOEs with R a = 2.75 n m were obtained. The optimized model is verified to be effective and further provides theory guidance and engineering application in the optical manufactory field of soft-brittle materials for HDOEs. The range of application materials for HDOEs is enriched, and the freedom of advanced optical design is broadened.