Abstract: Based on the variable-axes machine tool BKX-Ⅰ, developed in Beijing Institute of Technology, some relevant structural parameters of the machine tool's platforms are optimized regarding the tool's stiffness by the SQP process in MATLAB's optimization toolbox. After optimization, the horizontal stiffness is obviously enhanced. It was found that when the two platforms were both triangular in form, the largest horizontal stiffness value was obtained, and this provides a basis for the structural design of this type of machine tools.