The invention discloses a modification method for a salt-free low-alkaline deeply dyed type cellulose fiber fabric and a dyeing process for the salt-free low-alkaline deeply dyed type cellulose fiber fabric. The modification method is characterized by comprising the following steps: (1) preparation of chitosan oligosaccharide; (2) preparation of reactive chitosan oligosaccharide derivative; (3) modification of cellulose fibers with the reactive chitosan oligosaccharide derivative, and dyeing of a modified cellulose fiber fabric with reactive dyes in a dyeing machine, so as to obtain the salt-free low-alkaline dyed fabric. The method for modification of the cellulose fibers with the reactive chitosan oligosaccharide derivative and the dyeing process have the advantages that the binding strength between the chitosan oligosaccharide and the cellulose fibers can be obviously improved, the adsorption capacity of the obtained chitosan oligosaccharide modified cellulose fiber fabric for the reactive dyes can be obviously enhanced, the cellulose fiber fabric can be dyed under the salt-free low-alkaline condition to achieve a certain deep dyeing effect, the color fastness is improved, the problems of color tone, ring dyeing, color lake and poor deep dyeing property of the salt-free low-alkaline dyeing process can be solved, and the defects of hardening, stiffness, chemical fiber and the like on the surface of the fabric processed with the high molecular weight chitosan oligosaccharide can be overcome; in addition, the best dyeing effect can be obtained through optimizing the dyeing process.