The invention discloses a ferric oxide mesocrystal nano particle formed by crystal directional arrangement of ferric oxide nano particles being 2nm to 6nm, and having the size of 20nm to 80nm; the specific surface area of the ferric oxide mesocrystal nano particle reaches 56m<2>/g. A synthesis method of the ferric oxide mesocrystal nano particles comprises the following steps of step one, completely dissolving ferric nitrate into water, then adding ethyl alcohol and uniformly mixing to obtain a ferric nitrate mixed solvent solution; step two, adding the ferric nitrate mixed solvent solution in the step one into a hydrothermal reaction kettle, sealing and placing in the environment of 130 to 150 DEG C for reacting for 8 to 48h; step three, completely cooling a product obtained by the step two, putting the same into water for centrifugally washing for three times, and then putting into ethyl alcohol for centrifugally washing for three times. According to the ferric oxide mesocrystal nano particle as well as the synthesis method and an application method thereof, provided by the invention, a simple water and ethyl alcohol mixed solvent solution system being low in price is adopted, the high-purity and high-yield ferric oxide mesocrystal nano particle can be obtained by hydrolysis reaction, at relatively low temperature, of the ferric nitrate in the hydrothermal reaction kettle. By weighing energy constant calculation, the yield of the ferric oxide mesocrystal nano particle can reach 80 percent or above.