Abstract: 【Objectives】Many researches have involved in the relationship between nitrogen forms and host nutrition and disease severity, and the effects of nitrogen nutrition on plant secondary metabolites. Spot blight is one of thecommon diseases of Chrysanthemum morifolium, and flavonoids and phenolic acids are the major chemicalingrediants of Chrysanthemum morifolium against the disease. In order to provide references for nitrogen fertilization with low disease and high level of secondary metabolism of Chrysanthemum morifolium in field production,effects of different ammonium to nitrate proportions on the secondary metabolism and physiological indicators relating to spot blight of Chrysanthemum morifolium were studied in this paper. 【Methods】 Pot experiments were carried out in greenhouse at the experimental station of Nanjing Agricultural University. At the base of same nitrogen level,five proportions of ammonium to nitrate nitrogen were designed with four replicates.Plants were chosen with diagonal lines for regular scheduled survey of spot blight incidence, andthe method of leaf description and LI-3000C leaf area meter were used to determine the level of plant diseases,disease index and disease incidence. The contents of lignin, cellulose, soluble sugar, soluble protein, the activities of superoxide radicals (O-2), malondialdehyde (MDA), superoxide dismutase (SOD) and peroxidase (POD) antioxidant enzymes were determined, and the secondary metabolites chlorogenic acid, luteolin, 3,5-O-double caffeoyl quinic acid in the flowers and total flavonoid content in different parts of Chrysanthemum morifolium were measured. 【Results】The contents of lignin, cellulose, total soluble sugar, POD, phenylalanine ammonia-lyase (PAL) and O-2 are all highest at the NH+4-N/NO-3-N ratio of 25:75, while the content of MDA and SOD activity are relatively lower. The total flavone content maintains a relatively higher level at the NH+4-N/NO-3-N ratio of 25:75-0:100, and the contents of chlorogenic acid and 3,5-O-dicoffeoylqunic acid are 0.67% and 1.84% respectively, reaching the maxima at the NH+4-N/NO-3-N ratio of 25:75. The incidence of spot blight and the disease index gradually drop with the increase of nitrate nitrogen proportion, and reachs the lowest level at the ratio of 25:75. The correlation analysis shows that lignin, total soluble sugars, the activity of PAL and POD, O-2, chlorogenic acid, 3,5-O-double-caffeoyl quinic acid and total flavonoids of flowers are negatively correlated with the spot blight incidence and disease index, reaching a significant level of 0.01, while the SOD activity and MDA content are positively correlated with the spot blight incidence and disease index, reaching a significant level of 0.05. 【Conclusions】 The results show that the spot blight incidence and disease index of Chrysanthemum morifolium are closely related to physiological indicators and secondary metabolites, and the spot blight and secondary metabolism of Chrysanthemum morifolium are significantly related to NH+4-N/NO-3-N ratio and reach the lowest level at the ratio of 25:75.