Slow Tool Servo diamond turning is capable of effectively generating freeform optical surfaces or rotationally non-symmetric surfaces. However, many error sources, such as geometric and thermal errors of machine tool, and the forced vibration, dominate machining accuracy. In order to enhance fabricate accuracy, machining error compensatory approach was studied. Zernike polynomials are powerful in analyzing the relationship between the surface figure and the optical performance. So machining error was analyzed by Zernike polynomial fitting procedure, and the error compensation algorithm based on tool path correction was constructed. Experiments show that the slow tool servo diamond turning method can generate freeform optical surface, while the compensation method can obviously improve the machining accuracy.