To meet the requirements of large aperture optical elements for high precision manufacturing, a 3-axis Computer Numerical Control(CNC) milling machine was reconstructed into a 5-axis CNC one to implement the hardware improvement. The reconstructing process from the 3-axis machine to the 5-axis one was introduced, and a method to calibrate the parameters of the tool spindle was put forward. The method used a higher precision laser tracker to build coordinate systems, collect data and calculate geometric parameters, then to obtain the calibrating results of the rod length and swing angle for the tool spindle. Precision analysis shows that the angular precision is better than 10.000″ and the length precision is better than 0.040 mm. The proposed method can also be applied to similar tasks.