In order to achieve the requirement of drilling, milling and boring to a gearbox front housing at the same time when it was clamped one time, a set of automatic fast hydraulic special fixture used in a machining center for gearbox front housing was designed. Aimed at the structure of gearbox front housing, by taking ‘one surface with two pins’ positioning scheme, the selfcenter structure was designed and the structural theory was elaborated, so reliable positioning was ensured. Hydraulic clamping was used by the fixture, and the cutting force and the clamping force were calculated, then the corresponding hydraulic system was designed with chosen hydraulic cylinder of the fixture. Practice proves that the fixture has the advantages of simple structure and easy to operate, and the transition of baseline is avoided so as to ensure the machining accuracy and improve the machining efficiency. 