A method for acquiring spectrum shape of a gain flattening filter of a doped optical fiber amplifier comprises the steps of: measuring spectrum shapes at two gain point (H, L) of the doped optical fiber with invariable fiber length respectively; and acquiring various gain spectrums of the doped optical fiber with various fiber length and various population inversion level according to an expression: ErGain(λ,x,L′)=[ErLGain(λ)+[ErHGain(λ)−ErLGain(λ)]*x]*L′, Wherein Gain(λ) refers to the spectral function of gain, x is Δ′inv/Δinv which refers to change of population inversion level, and L′ is set as proportion of doped fiber length. Gain spectrums of the doped optical fiber with various fiber length can be acquired by measuring spectrum shapes at two gain point (H, L) of the doped optical fiber in invariable fiber length and applying change rule of gain spectrum of the doped optical fiber in different population inversion level, which improves the flexibility for design of amplifier.