Xinjiang apricot fruits (Saimaiti) were sprayed with 0.05% chitosan oligosaccharide(COS) with molecular weight 5000 Da in four periods:fruit setting period,fruit enlarging period,color turning stage and 48 h before1 picking.The post-harvest apricot fruits after being innoculated were stored at 4 ℃,90%-95%RH.The lesion diameter and inoculation incidence of apricot fruits inoculated with Alternaria alternata.The activities of defense related enzymes including phenylalanine ammonia-lyase (PAL),β-1,3-glucanase (GLU) and chitinase (CHT) and the contents of lignin,hydroproline-rich glycoprotein(HRGP) were measured regularly.Effects of preharvest chitosan oligosaccharide-induced resistance to black spot in fruits and contents of HRGP and lignin in cell wall was studied.The results showed that disease incidence and lesion diameter were lower in chitosan oligosaccharide-treated fruits compared with their respective controls at the end of storage.The activities of PAL,CHT,GLU and the contents of lignin and hydroproline-rich glycoprotein in chitosan oligosaccharide-treated fruits were showed the change trend:rise after the fall，and was significantly higher than their respective controls in the same period.The activities of PAL,CHT,GLU and the contents of lignin and hydroproline-rich glycoprotein in chitosan oligosaccharide-treated fruits reached the peak at 21^th day,28^th day,21^th day,28^th day,14^th day.Moreover,all of peak were 12.17%,78.22%,31.41%,34.81% and 77.44% higher than their respective controls.The above results indicated that pre-harvest chitosan oligosaccharide can induce the disease resistance of apricot fruit treated by improving the disease related protein,HRGP and the content of the cell wall.