In this paper, we demonstrate a high power fiber oscillator based on a pair of 20 μm /400 μm large mode area double cladding fiber Bragg gratings fabricated by a femtosecond laser together with a phase mask. The reflectivity of the high-reflection FBG is more than 99%, and that of the low-reflection FBG is about 10%. The slope efficiency of the fiber oscillator is 77.9%, and the maximum power output is more than 3.2 kW. To the best of our knowledge, the kW level class fiber oscillator using FBGs fabricated by a femtosecond laser has never been reported in our nation before, and our research works fill the vacancy in this field, which is meaningful in fabrication of high power FBGs.