1. A method for an apparatus to establish a scattering bar rule for a mask pattern for fabricating a device, the method comprising: an image simulation module of the apparatus establishing at least one image simulation model according to the mask pattern and a process reference set used for fabricating the device based on the mask pattern; the image simulation module applying a plurality of scattering bar reference sets to the image simulation model so as to generate a plurality of simulation images, respectively; a screening module of the apparatus screening a portion of the simulation images to be a plurality of candidate layouts according to a screening criterion; and a selection module of the apparatus selecting one of the candidate layouts to be a pattern layout according to a selection rule, and determining the scattering bar reference set corresponding to the pattern layout to be a scattering bar rule of the mask pattern.