Abstract: This paper designs ffgg★protocol which is similar toffggprotocol constructed by Millen.They have cryptographic properties in commen. Then,the protocol is analyzed by using constructing attack based on generalized strand space model ,and it is proved that the protocol is secure under non-type-flaw attacks .After ffggprotocol is analyzed by Millen using Pulson`s inductive approach,it is secure under non-type-flaw attacks,either. It is indicated that constructing attack has limitation. Finally,some improved measures are put forward,and type-flaw attack is found by using amendatory constructing attack inffgg★protocol.