A flat high gain L-band gain-clamped erbium-doped fiber amplifier (GC-EDFA) by using backward C-band amplified spontaneous emission (ASE) is proposed. It uses two stages of EDFs pumped forward as gain media, and a broadband chirp fiber Bragg grating (CFBG) to reflect the backward C-band ASE back into the loop to control the gain, which avoids spectral-hole and relaxation oscillation due to no controlling laser exists. Compared with other similar GC-EDFAs, the proposed structure has higher and flatter clamped gain in L-band because of its optimal structure -- the length of EDFs and pump power, pump scheme, etc.. The gain is above 23 dB, the bandwidth of 3 dB is more than 36 nm, and the input signal saturation power arrives -15 dBm.