For improving efficiency in fabrication of multi-linewidth integrated optical device, a new fabrication method based on defocus laser writing, namely, a method that the laser does not focus on the surface of photoresist during fabrication, was proposed, and the corresponding mathematical model of linewidth was set up, too. This model firstly supposes that the transformed laser beam through objective lens keeps Gaussian energy contribution in arbitrary defocus plane, then describes the influence of laser spot outline on exposure energy contribution during laser dynamic scanning. The model is named “dynamic Gaussian model”, which is correlative with light power, defocus amount, scanning velocity of laser beam, exposure energy threshold of photoresist and other fabrication parameters. The verification was carried out in self-developed polar laser pattern generator. It shows that the dynamic Gaussian model is in better agreement with the experimental result and is much better than the static Gaussian model,which ignores the movement of laser spot.