An improvement of state failure rate model for power transmission and transforming equipment is presented，which considers model parameters changing with time.Besides，in order to improve solution of model parameters a new parameter estimation method called all-state integration（ASI）is proposed.ASI is used to study on an integral health process which is with time tag of each device，and follows state information of integral process.So model parameters are marked with service age.Finally，improved model parameters and the failure rate of different equipment with different service age are calculated with ASI method in an example.Compared to the original model and the calculated results of inversion method，the results calculated with ASI are more closer to the actual statistical failure rate.