Since China's enforcement of building energy saving standards, external insulation technology has been applied in a mount of buildings. However, four common defects of it, namely crack, shedding and hollow, leakage, frost and condensation, had gradually appeared. This study tries to find out the critical construction techniques of preventing these defects which are closely related to quality in order that engineers pay more attention to them during construction. Literature research and field investigation to engineers and technicians are adopted. Designed questionnaires, including 61 kinds of detailed defects causes and relevant preventive measures, are used in the survey. Through statistical analysis of survey data, the mesh, plastering mortar configuration, etc. 10 detailed causes are suggested as main defect reasons according to harm grade. Meanwhile, corresponding 15 highly effective construction technical countermeasures are put forward finally.