Besides identifying them directly from wind tunnel tests, aerodynamic admittance functions of bridge sections could be expressed using alternative methods. Through assuming Wanger function to be equivalent to Kussner function, Scanlan proposed using bridge flutter derivatives to express aerodynamic admittance functions. Hatanaka et al proposed using ‘equivalent’ Theodorsen functions to express aerodynamic admittances. Although both of them simplified identifying aerodynamic admittances, there were logical problems in the two methods. Here, flutter derivatives and aerodynamic admittance functions of a flat plate section and a rectangular section with a aspect ratio of 4 were identified with wind tunnel tests. Then, through comparing the measured aerodynamic admittances with those calculated using the above two methods, irrationality of the two methods was verified. The results indicated that with increase in the reduced frequency, the difference between the calculated aerodynamic admittances using the method of equivalent indicial functions and the measured ones gradually increases and tends to a limit value due to ignoring higher order motion modes, this method is applicable only when the wave length of fluctuating wind is much larger than the characteristic length of bridge sections; the aerodynamic admittances calculated using the equivalent Theodorsen functions are close to the measured ones within a lower frequency range, while they have a periodic fluctuating within a higher frequency range; for rectangular section of bluff bodies, this fluctuating becomes more significant; this fluctuation is due to adopting a certain ‘equivalent’ Theodorsen function to describe objects’ aerodynamic performance, once Theodorsen function changes, but its component functions remain unchanged to cause a logical mistake.