Photosynthesis is the basis for plant survival, and the mechanism of photosynthesis and chloroplast development are hotspot in the field of life sciences. Leaf color mutants are often accompanied by abnormal chloroplast structure and blocked photosynthetic pigment synthesis. Therefore, the study of leaf color mutants can provide experimental data and theoretical support for photosynthesis and photomorphogenesis. In this study, maize etiolation mutant et9 (etiolation 9), which was screened from maize inbred line PH6WC by 2.48Gy fast neutron irradiation, was used as the material. Through phenotypic identification, chlorophyll content determination in leaves, chloroplast structure microscopic observation, photosynthetic characteristics analysis, et9 plant height and ear height were significantly reduced compared with the wild type, and the flag leaf length, flag leaf width and the third leaf width were significantly reduced. The tasseling, pollen shedding and silking stages were delayed by 10-12 days compared with the wild type. The contents of chlorophyll a, chlorophyll b and total chlorophyll were significantly lower than those of wild type. The chloroplast structure is loose, the distribution of thylakoids is chaotic, and the number of basal grains is small; the net photosynthetic rate, stomatal conductance and transpiration rate were significantly lower than those of the wild type, and the intercellular carbon dioxide concentration was significantly increased. The chloroplast fluorescence parameters were significantly lower than those of the wild type except for photochemical quenching. Genetic pattern analysis showed that the yellowing phenotype was controlled by a nuclear recessive gene, named Zmet 9 (Zea mays etiolation 9). The F2 segregating population was constructed by crossing it with maize inbred line B73, and the mutation site was preliminarily located in the 2 Mb range of 20 - 22 Mb on chromosome 9 of maize by BSR-seq method. Four pairs of KASP markers and two pairs of Indel markers were further developed in the initial mapping interval, and about 1100 mutant phenotypic plants were used for fine mapping. Finally, Zmet 9 was finely mapped to a region of about 160 kb between KASP19 and 2040 on chromosome 9 of maize. This interval contains seven candidate genes, of which Zm00001d045384 encodes an iron superoxide dismutase, and the leaf color bleaching phenotype occurs after the mutation of homologous genes FSD2 and FSD3 in Arabidopsis. We speculate that Zm00001d045384 may be a candidate gene for Zmet 9.