A fast computation method for broadband echo characteristics was proposed based on the planar element method and by using Chebyshev polynomial interpolation. Introducing the amplitude and phase separation method, the scattering sound field of each planar element was decomposed into fast and slow fluctuating functions varied with frequency. For the function varied slowly with frequency, the Chebyshev approximation theory was used to obtain the precise calculation of each frequency point in broadband, when the scattered sound of Chebyshev nodes was given by the planar element method. The broadband echo characteristic of the target was calculated by the combination of the result of Chebyshev polynomial interpolation and fast fluctuating function. The method can avoid high cost of time in scattering field calculation, because it is not necessary to compute point by point in the frequent domain. Compared with the traditional planar element method, this method can compute the broadband echo characteristics of underwater target quickly and accurately.