In this work,we investigate the waveguide fabrication and the mechan ism of femtosecond laser direct writing in borosilicate aluminum magneto-optical glass.We compare the charact eristics of the magneto-optical waveguides fabricated in two kinds of glasses with different concentrations of T b³⁺ ions,i.e.,TG20and TG28glasses, and systematically study the dependence of the guiding modes and Verdet consta nt of waveguides on femtosecond laser pulses′ polarization and energy.The results show that the c ircularly polarized femtosecond pulses have lower fabrication threshold energy and can introduce larger in crease in the refractive index change inside magneto-optical glass,compared with the linearly polarized pulses.It con f irms that the refractive index change in borosilicate aluminum glass mainly comes from the 3- or 4-photon absorption of ionic bonding.The magneto-optical coefficient measurement results prove that in TG28glass,the power and polarization state of the incident pulses have small influence on Verdet constant of waveguides.The decre ase in Verdet constant of magneto-optical waveguide is less than 15% of the value in bulk TG28glass,whe reas the decrease can reach 70% in TG20glass.Therefore,TG28glass is more appropriate to fabricat e magneto-optical microstructures by using femtosecond laser direct writing than TG20glass.By careful selectio n of the dopant concentration in the magneto-optical glass the opt ical microstructures with high magneto-optical coefficient and high refractive index modulation may be fabrica ted.