Abstract: 【Objective】The content of ascorbic acid during the development of guava fruit was determined and the expression characteristics of ascorbic acid synthase gene in guava were analyzed, which provided theoretical reference for further exploring the regulation mechanism of fruit ascorbic acid synthesis and breeding guava varieties with high quality.【Method】The fruit of guava variety Pearl was used as the research object to determine the content of ascorbic acid in the flesh(fresh weight) and the total amount of ascorbic acid in single fruit during the development of different fruits(10-130 d after anthesis). The ascorbate synthesis pathway enzyme genes were obtained from the genomic data of guava, and their expression characteristics during the development of fruit were detected by real-time fluorescence quantitative PCR.【Result】The ripening period of Pearl guava variety was 130 d, and the growth rate showed two peaks, which appeared in the young fruit expansion stage and the large fruit expansion stage, respectively. The color of the fruit peel changed from dark green to light green, and the color change was the most obvious at 110 d after flowering. The content of ascorbic acid in guava flesh remained at a high level in the early stage of fruit development and ripening(10-40 d after flowering), especially at 30 d after flowering, the content of ascorbic acid in guava flesh reached 132 mg/100 g, and slightly decreased but remained stable at more than 102.73 mg/100 g at 40-100 d after flowering. The ascorbic acid content decreased slightly with the rapid expansion of fruit in the late ripening period, to 84.39 mg/100 g, but the total ascorbic acid content of single fruit continued to increase. A total of 13 genes encoding enzymes related to fruit ascorbate synthesis pathway were obtained from guava genome data, and these genes encoded complete L-galactose pathway related enzymes, among which 6 genes were concentrated in chromosome 3, suggesting that chromosome 3 was the key chromosome for vitamin synthesis in guava fruit. Real-time fluorescence quantitative PCR results showed that except for the irregular expression of phosphomannose isomerase gene(PgPMI1) and phosphomannomutase gene(PgPMM1) during fruit development and maturation, the expression of the other 11 L-galactose pathway related enzyme genes was up-regulated in the middle stage of fruit development, and down-regulated in the later stage. The content of ascorbic acid in fruit was significantly positively correlated with the expression of GDP-mannose pyrophosphorylase gene(PgGMP1) and GDP-mannose-3',5'-epimerase gene(PgGME2)(P<0.05). 【Conclusion】The accumulation of ascorbic acid in guava fruits is closely related to the integrity and expression level of the synthesis pathway genes. It is speculated that the L-galactose pathway composed of 9enzymes has a complex molecular regulatory network. The 13 members of L-galactose pathway obtained by mining can be used for subsequent breeding of excellent guava varieties and transcriptional regulation mechanism research.