Based on the Morphed Integrated Microwave Imagery data from Cooperative Institute for Meteorological Satellite Studies of University of Wisconsin and the best track data from Joint Typhoon Warning Center,the tropical cyclones(TCs) with secondary eyewall over the Northwest Pacific are examined.During 2005-2014,35 TCs with secondary eyewall are identified.The composited studies are performed to reveal the differences in environmental fields and initial structures between TCs with and without secondary eyewall (between fast and slow samples of secondary eyewall formation).Results show that both the environmental fields and initial structures can modulate the secondary eyewall formation.Specifically,the larger relative humidity of the environment field and the higher sea surface temperature,the easier the secondary eyewall will be formed.Furthermore,the formation rate of secondary eyewall is closely related to its own structure.A larger scale initial vortex leads to a faster secondary eyewall formation,father location of outer eyewall from typhoon center,longer duration of eyewall,and more dramatic intensity change before and after eyewall replacement.