Optical free-form surfaces have been widely studied and applied because of their good performance, and ultra-precision manufacturing with round nose cutting tool is an effective method for machining such surfaces. Because of the direct interaction between the workpiece surface and the cutting edge, the geometrical parameters of the round nose cutting tool directly affect the shape accuracy and the surface topography of the workpiece and need to be measured accurately and comprehensively. At present, the main method to measure the geometrical parameters of round nose cutting tool involves the method using machine vision, but this imposes some limitations, such as the strong dependence on the measurement angle and the inability of measuring multiple parameters at the same time. In this work, we built a focus variation system to make three-dimensional measurement of round nose cutting tool and used this system to obtain multiple geometrical parameters in a single measurement. The types of geometrical parameters and the extraction process are discussed in detail. And a standard step height is measured to verify the high accuracy of the measurement system. The repeated measurement results show that the standard deviation of the nose radius measurement can reach hundred-nanometer scale and the relative standard deviation can reach 0.028%.