By changing the laser pulse width， the optical waveguide is directly written in boro-aluminosilicate glass， and the effects of the pulse width on the section morphology of the optical waveguide， the size of the mode field and the transmission ratio of the directional coupler are studied. In the experiment， a single mode/multi-mode critical power laser with a pulse width of 239 fs is used to fabricate the waveguide. The cross section size of the waveguide changes from 5.4 μm×4.2 μm （239 fs） to 5.1 μm×2.4 μm （700 fs）. The size of the mode field changed from 6.2 μm×6 μm （239 fs） to 5.8 μm×4.8 μm（700 fs） and the transmission modes were single mode. The shape of the mode field gradually changed from round to ellipse. The transmission ratio of the directional coupler changes from 97.8% （239 fs） to 70.2% （700 fs） in the horizontal polarization and from 99.2% （239 fs） to 79.7% （700 fs） in the vertical polarization. Experiments have found that the laser with a pulse width of more than 600fs is not conducive to the preparation of waveguides with stable performance.