To explore the critical period and key enzymes of organic acid accumulation, and reveal the cause of low acid in fruit, we used the ‘Golden Delicious’ apple and its excellent strain (SGP1) as materials to determine the components and contents of organic acids and the activities of metabolismrelated enzymes during fruit development, then analyzed their variation law and correlation. The results showed that: (1) during the fruit development, the content of organic acids in ‘SGP1’ was significantly lower than that of ‘Golden Delicious’, which was only half of that of ‘Golden Delicious’ at maturation stage. The organic acids of ‘SGP1’ were mainly composed of malic acid, quinic acid, tartaric acid and citric acid, with quinic acid as the main component at young fruit stage and malic acid as the main component at maturation stage. (2) The malic acid content of ‘SGP1’ was significantly lower than that of ‘Golden Delicious’, whose variation law was opposite to that of ‘Golden Delicious’ at young fruit stage and fruit enlargement stage, and the critical accumulation stage and rapid decline stage were earlier than that of ‘Golden Delicious’. The change trend of other acid components in ‘SGP1’ and ‘Golden Delicious’ was basically the same, and the acid content at young fruit stage of ‘SGP1’ was significantly higher than that of ‘Golden Delicious’, while there was no significant difference at maturation stage. (3) The activities of malic acid metabolismrelated enzymes in ‘SGP1’ were significantly higher than those in ‘Golden Delicious’ at young fruit stage, and were equal to or significantly lower than those in ‘Golden Delicious’ at maturation stage; The changes of MDH activity in young fruit stage and ME activity in maturation stage of‘SGP1’and ‘Golden Delicious’ were opposite. (4) Correlation analysis showed that the malic acid accumulation of ‘SGP1’ in young fruit stage was extremely significantly positively correlated with PEPC and VHA, while there was an extremely significantly negative correlation between the malic acid accumulation and activities of MDH, PEPC and VHA in ‘Golden Delicious’. At fruit enlargement stage, malic acid accumulation of ‘SGP1’ was extremely significantly negatively correlated with MDH and PEPC activities, and significantly positively correlated with PEPCK and VHA activities, while there was a extremely significantly or significantly negative correlation between the malic acid accumulation and activities of PEPC, ME and VHP in ‘Golden Delicious’. The malic acid accumulation in ‘SGP1’ and ‘Golden Delicious’ at maturation stage was extremely significantly or significantly negatively correlated with MDH, PEPC, ME and VHP activities. It was found that ‘SGP1’ was a excellent variation strain of ‘Golden Delicious’ apple with low acid content, which was dominated by malic acid. The types and activities of enzymes that mainly regulate malic acid accumulation were different from those of ‘Golden Delicious’, resulting in the low acid quality of ‘SGP1’. This study laid a foundation for further exploring the formation mechanism of low acid in fruits and cultivating new varieties with high sugar and low acid.