The failure information of the cooling system of machining center is fuzzy frequently. The normal analysis method of failure mode critically only considers the failure probability and serious degree. When the testing and the maintainability of failure are considered, the failure mode criticality can not be accurately analyzed by FMECA. The fuzzy theory was proposed to analyze the failure mode criticality. It comprehensively included the failure probability, the failure serious degree, the testing and the maintainability of failure, which could carried out the qualitative question quantitatively. The failure mode criticality of cooling system of a machining center was calculated to determine the direction of improving reliability. The results showed that the criticality of jam stem was serious, which was consistent with the facts.