Adding hydrostatic guideway composed of diameter enlargement platform, hydrostatic support base etc;using specific oil supply technology; adding lengthened machine tool, exchanging driving shaft of machine bed, forming a new part for machine tool together with spare parts of original hobbing machine body, the enlargement of the diameter of gear processed by hobbing machine was implemented. Taking 5343 type hobbing machine as example, the selection & design of relevant parameters of hydrostatic guideway for diameter enlargement transformation were introduced as well as the design of main spare parts.