With construction of the growing amounts of energy saving buildings, there are defects cracks in external wall insulation because of lack of construction and management experience. The purpose of this study is to find out main causes of cracks of external wall insulation covered by ceramic tile, to conduct construction industry to control engineering quality by paying attention on them. On basis of document study, field investigation to construction workers, supervisors, project managers, both material and construction causes of cracks are studied by investigation and designed questionnaires, including 32 detailed reasons and their harmful levels. Meanwhile, the base data from them is analyzed, and Pareto charts of causes of crack are drawn accordingly. The result shows that grid cloth and plaster mortar are the main material reasons, and that plaster mortar construction, grid cloth laid, detail treatment, configuration of plaster mortar are the main construction reasons.