This paper presents a new simulation system for ultra-precision diamond turning of optical free-form surfaces based on fast tool servo (FTS). The functions including evaluation and analysis of optical free-form surfaces manufacturing errors, optimization for tool path, etc, are proposed. The simulation modules and functional structure of the system are designed in accordance with the functions. The errors in the process of machining can be found in advance through the simulation. Then the NC code will be modified timely. The proposed method can be used to improve product quality, shorten the development cycle, reduce costs and optimize machining process.