Three types of longperiod ground motions including farfield harmoniclike ground motions, nearfault forwarddirectivity ground motions, and nearfault flingstep ground motions were selected as objects. Firstly, based on the analysis of correlation between the maximum instantaneous input energy of longperiod ground motion and the maximum displacement response of a SDOF system, the concept of instantaneous input energy ratio was proposed, and the structural failure modes under different types of longperiod ground motions were discussed. Then, one 12layer RC frame structure was taken for example, and the controlled IMF components of longperiod ground motions for the elasticplastic seismic response were analyzed based on HilbertHuang Transform (HHT). Besides, the instantaneous energy curves and accumulative energy curves for different types of longperiod ground motions were investigated. And on this basis, the concepts of valid longperiod ground motion, valid peak and energy gradient were proposed, and the action mechanisms of longperiod ground motions on highrise structures were studied from the perspectives of frequency and time domains, respectively. The results show that the valid peaks of longperiod ground motions are larger than those of ordinary ground motions, and it reveals the internal mechanism which causes the seismic responses of highrise structures under longperiod ground motions are larger than those under ordinary ground motions; the analysis of energy gradients shows that the energy release characteristics of different types of longperiod ground motions are obviously different, and it reveals the action mechanisms of longperiod ground motions. For the farfield harmoniclike ground motion, it has the pulse cyclic action mechanism which is easy to cause the cumulative damage failure; for the nearfault forwarddirectivity ground motion and nearfault flingstep ground motion, they have the pulse impact action mechanism which is easy to cause the first exceed failure.