Abstract: There is a new type of overvoltage phenomenon in half-wave length transmission line（HWLL） caused by power fluctuation（PF）, so this paper defined it as power fluctuation overvoltage（PFO）, which belongs to temporary overvoltage category. The time scale of PFO extends from microsecond or millisecond of traditional overvoltage to second level in the case of single-phase instantaneous grounding. The amplitude of PFO is always high, the duration is long, and the coverage on HWLL is wide. It will become a new problem of half-wave length AC transmission technology. Firstly, the unique transmission characteristics of HWLL were analyzed deeply with transmission line theory. Next, based on a large number of electromagnetic transient simulations, the PF characteristics of UHV HWLL were obtained. Then based on the characteristics, mechanism and characteristics of PFO were elaborated. The convention of generally not using arresters limit temporary overvoltage should be broken for HWLL. Thus, considered from overvoltage and equipment tolerance, the mitigations of PFO are proposed for UHV HWLL, which are configuring metal oxide arresters（MOAs） differentially along HWLL and with using of inputting high speed grounding switch（HSGS） along the line during fault period, generator tripping after fault, or limiting reclosing time.