In this paper, the finite element method is used to simulate the energy, electric field and effective refractive index changes of the modes in the microfiber before and after coating. The transmission characteristics of HE₁₁, TE₀₁, HE₂₁ and TM₀₁ modes in the microfiber and the interaction principle with the platinum film are analyzed. The microfiber was fabricated by etching the optical fiber with buffer oxide etchant, and the platinum film was coated by ion sputtering to obtain a microfiber device with a diameter of 13.2 μm and a platinum film thickness of 40 nm. Its saturable absorption properties were tested, the modulation depth and saturation intensity were 0.57% and 0.8 MW/cm², respectively. An all-fiber mode-locked laser was fabricated, its mode-locked threshold is 180 mW. The mode-locked pulse repetition rate is 17.93 MHz, the pulse width is 103 ps, the center wavelength is 1031.6 nm, and the full width at half maximum is 3.5 nm.