Abstract: Because of the big mass of moving parts, three motors are needed on parallel machine tool to drive those parts, and the dynamic balance of the machine tool is needed during moving. Three counterweight pulleys are therefore provided for relevant branch chains to implement the spatial dynamic balance via the parallel mechanisms positioned between the moving and resting platforms for every branch chain. By means of theoretical derivation and practical calculation, the counterweights needed for two branch chains are 31.09 kg and 57.69 kg, and that for the moving platform is 157.36 kg. Simulation results showed that the maximum force output of drive rod is about 10N after dynamic balance-it is less by far than before dynamic balance: 1100 N. A satisfactory balancing result is thus obtained.