Abstract: The combination of dam heel concrete and bed rock, together with the closure of cracks in the bed rock near contact area always exert great influence to seepage and structural safety of arch dams. The research shows that dam heel concrete of the arch dam on wide valley is likely to crack, and avoidance of harmful temperature and water level is of importance to crack prevention. As well, tangential unsymmetrical deformation of arch dam is helpful in dam performance evaluation. When it is found that unsymmetrical deformation is divergent, dam body and dam foundation may be damaged which calls for high attention. Comprehensive diagnosis with monitoring data and counter-measures should be carried out to prevent accidents.