With the configuration of two-stage amplifying, the high power fiber amplifier with gain clamping and gain flattening is constructed. It makes gain flattened and gain clamped using fiber loop mirror (FLM) combined with two fiber Bragg gratings (FBGs). The output performance of the amplifier is tested, which shows that the maximum output power is 1.399 W and the gain fluctuation is reduced to ±0.75 dB in the range from 1535 nm to 1547 nm. Under the conditions of inputing small power and high power signals, the gain control characteristics of amplifier are researched, respectively. As the input power of channel 2 changing from -33.7 dBm to -2.5 dBm or -14.8 dBm to 16.4 dBm, the excursion ranges of channel 1 can be 0.04 dB and 0.06 dB, respectively.