Geometry parameters and lengths of legs often have close relations with the singularity of a Gough-Stewart platform. When these parameters are improperly selected,the platform may have hunt singularities. The relations among the geometrical parameters and the leg lengths when in hunt singularity for 3-3、6-3、 6-6 Gough-Stewart are derived according to their geometrical constrains. The obtained relations can be used as a geometrical criterions,for a Gough-Stewart platform with given geometrical parameters,to determine if the mechanism may have or close to Hunt singularity in its workspace. Two experiments are performed,which validates the hunt singularity criterions.