There is some disagreement on the mechanism and test results of concrete resistance to freeze/thaw action. The factors influencing damage of concrete were tested with different fly ash dosages, manner of freeze/thaw action and constitution of concrete. The results show that the mechanism of failure of concrete with strength grade 50 MPa under freeze/thaw cycle is mainly icing pressure. The research conclusions can be used as a base in designing of concrete with fly ash used in some important concrete engineering like Nanjing Second Yangtze River Bridge.