FeNi alloys were synthesized by hydrothermal reduction method. The effects of different annealing temperatures on magnetic properties and microwave absorption properties were investigated. As the annealing temperature increases from 150 °C to 700 °C, the saturation magnetization of FeNi alloys increases from 83.61 to 103.24 A·m2·kg−1, and the coercivity decreases from 12.18 to 0.68 kA·m−1. FeNi alloy changes from semi-hard magnetic behavior to typical soft magnetic behavior. The optimum reflection loss (RL) of the FeNi alloy (700 °C) is −28.36 dB at 3.84 GHz. Especially, the effective absorption bandwidth (RL < −10 dB) of FeNi alloy (700 °C) is up to 3.26 GHz. These results suggest that the FeNi alloys could be a desirable S-band operational microwave absorber.