A tunable, gain-clamped (GC) double-pass Erbium-doped fiber amplifier (EDFA) using only one fiber Bragg grating (FBG) has been demonstrated, which solves the problem existing the conventional GC-EDFA using two FBGs, in which the clamped-gain is very difficult to be tuned. In the new GC-EDFA, the lasing oscillation for clamping the gain is produced between a FBG and a fiber reflection mirror, and a variable optical attenuator (VOA) is used to change the loss of the laser, which is filtered solely from a narrowband filter for tuning the clamped-gain, however it does not change the signal power directly. Meanwhile, the double-pass configuration enhances efficiently the gain, therefore, compared with the single-pass configuration, the maximum possible input signal power for gain-clamping is greatly extended. Furthermore, the FBG can depress the strong backward amplification spontaneous emission in double-pass configuration, so it can reduce the noise figure a certain extent. Finally, a gain-tunable GC-EDFA with a wide dynamic input power range is demonstrated.