Provided is a brace apparatus for a transformer tank and a method for determining the length thereof. The brace apparatus is mounted on a transformer tank so as to reinforce the transformer tank. The brace apparatus has a brace main body forming the exterior thereof, and in order to block the occurrence of resonance in an inner space of the brace main body, the length of the brace main body is set to be at a value at which resonance does not occur in the inner space, or a partition plate is provided to the brace main body so as to block the occurrence of resonance. In present invention, the length of the brace apparatus may be set by means of a simple configuration, thereby enabling the prevention of noise from occurring due to resonance.