A linewidth stabilizing method for defocusing laser direct writing is proposed for further improvement of line quality of diffractive optical elements. The writing power and defocusing amount are synchronously adjusted to set photoresist threshold at the position where the linewidth variation rate is small, thereby the linewidth sensitivity to variation of actual exposure dose and photoresist threshold is weakened, and then the linewidth stability during defocusing laser direct writing is guaranteed. Both the power control model and linewidth model of the proposed method are established with defocusing amount as the unique variable. The verification of linewidth model was carried out on CCD with laser wavelength of 632.8 nm and objects of numerical aperture 0.1, and it shows that the experimental result is in good agreement with that by theoretical model. The proposed method is of great significance for fabrication of high-stability diffractive optical elements.