The symmetrical optical waveguide structures are fabricated in Er 3+ -doped aluminosilicate glass using 800 nm femtosecond laser writing in kHz repetition frequency regime. The impact of writing parameters on the waveguide preparation with a 10× microscope objective in the longitudinal writing scheme was studied in detail. The experimental results show that, under a fixed pulse energy value of 20 μJ, the waveguides can be realized with the scan speed from 20 to 160 μm/s, and with a fixed scan speed value of 160 μm/s, the waveguide can be prepared under an inscription energy range of 5−30 μJ, indicating that the waveguide can be realized in a wide range of writing parameters. The near-field mode intensity image shows that the waveguide region possesses excellent light guiding properties. Based on the refractive index profile reconstruction method, the maximum refractive index increase in the waveguide region is estimated to be about 5.5 × 10 −4 . The lowest propagation loss of the waveguide was measured to be about 1.51 dB/cm by the waveguide side scattering method. The microfluorescence spectra show that the gain characteristics of the waveguide region are well maintained after laser processing. This work shows that the fabrication of embedded optical waveguides using ultrashort pulse laser in Er 3+ -doped aluminosilicate glass has strong feasibility and great potential to create active gain devices in the field of integrated photonics and all-optical communication.