The excitonic insulator predicted 50 years ago is a correlated electron phase of narrow-gap semiconductor, and a gap spontaneously appears analogous to a Bardeen-Cooper-Schrieffer (BCS) superconductor. In this work, we find Ta₂Pd₃Te₅ is not only a predicted quantum spin Hall insulator[1], but also an excitonic insulator candidate, similar to InAs/GaSb quantum wells[2]. Different from a power law relationship between temperature and resistivity at low temperature, which may be originated from the edge state, a metal to semiconductor transition is observed in the resistivity at nearly 350 K, suggesting the open of gap, further confirmed by specific heat jump and susceptibility measurements. The low-temperature x-ray diffraction and transmission electron microscope measurements illustrate this transition is not a structure transition or charge-density-wave transition. Moreover, the gap below the transition increases with decreasing temperature, confirmed by both transport measurements and ARPES measurements. With tuning the Fermi level in nanodevice by back gate or electron/hole-doping materials, the paw law behavior/correlated strength and  gap can be well controlled. All these behaviors imply Ta₂Pd₃Te₅ is an excitonic insulator.