A fiber micro collimator with adjustable mode field diameter is proposed to address the problem of single output modefield diameter in traditional fiber collimators. The single mode fiber is combined with different 2-segment multimode fibers toform a "sandwich" structure. By adjusting the length of the middle section of the optical fiber, the mode field diameter can be adjusted arbitrarily, and a simulation experimental model has been constructed to prepare multiple fiber micro collimator sampleswith different sizes. The experimental results show that the structure can achieve adjustability of the output mode field diameter inthe range of 20~40 μm in the 1 550 nm wavelength band. The preparation method of this fiber collimator is simple, compact instructure, and can flexibly adjust the mode field diameter of the light field according to the needs to adapt to different applicationscenarios.