The invention discloses a locating method for machining of a gearbox housing. The locating method comprises the steps that two bearing holes of the housing are firstly located in a lower reference locating pin and a lower rotating limiting pin on a machining platform, and the housing is supported by a floating supporting device in a floating manner; then an upper reference locating pin and an upper rotating limiting pin are matched in the bearing holes in the upper side wall of the housing, therefore, the state of the housing is adjusted, and the supporting height of all supporting points of the floating supporting device is automatically adjusted so that the floating supporting device can closely abut against the initial locating face of the housing; then the height of all the supporting points of the floating supporting device is fixed, so that the housing is stably supported; and the upper reference locating pin and the upper rotating limiting pin are moved away, the bearing holes and the combination face on the housing can start to be machined, and the combination face and the bearing holes are the reference locating face and reference locating holes in the subsequent machining process. The coaxiality of the bearing holes in the machining process of the gearbox housing can be obviously improved, then unevenness of the machining allowance is lowered, and the machining precision of the housing is obviously improved.