Pitaya fruit is a characteristic tropical fruit of Hainan province, mainly cultivated in the red soil of Hainan province. The exchangeable magnesium is lost throughout the year, leading to magnesium deficiency in some orchards, which affects the growth and yield of pitaya fruit. Magnesium is involved in photosynthesis and other metabolic processes. Therefore, this study took the main domestic variety of pitaya Dahong as the experimental material, using hydroponic experiments, set four magnesium concentration gradients of 0, 0.5, 2.0, 4.0 mmol/L, to determine the changes in the apparent morphology, biomass, and the activity of enzymes related to crassulacean acid metabolism in pitaya fruit. The results showed: (1) Different magnesium concentration treatments had a significant impact on the length, width, thickness, stem thickness, and fresh weight of the tender stems of pitaya fruit. When magnesium is deficient or excessive, the length, width, thickness, stem thickness, and fresh weight of the tender stems are also significantly less than those treated with 2.0 mmol/L. Magnesium deficiency can significantly reduce the length, width, thickness, stem thickness, and fresh weight of pitaya fruit. The condition of 2.0 mmol/L magnesium concentration is most suitable for plant growth; (2) The activity of PEPC first increased and then decreased with the increase of magnesium concentration. The activity of PEPC at 4 mmol/L magnesium concentration was lower than at 2.0 mmol/L but higher than at 0.5 mmol/L and 0 mmol/L treatments. after 84 days, under the condition of 2.0 mmol/L magnesium concentration, the PEPC activity in tender and old stems was the highest among other treatments, at 90.44±1.40 and 92.77±0.67 nmol/(min·g), respectively, and its initial CO₂ fixation ability was much higher than other treatments; The activity of NAD-MDH in tender stems decreased with the increase of magnesium concentration. With the extension of treatment time, the impact of magnesium concentration on NAD-MDH gradually appeared, first observed in tender stems and then gradually affected the old stems. The maximum activity of tender and old stems treated with magnesium deficiency after 84 days was (12631.82±286.04)nmol/(min·g), and (10500.16±108.34) nmol/(min·g), respectively; Under the treatment of 2.0 mmol/L magnesium concentration, the activity of NAD-ME in both tender and old stems of pitaya fruit was maintained at a high level, with an overall average of 28.41~ 65.87 nmol/min/g. (3) Magnesium deficiency had a significant effect on the enzyme activities of PEPC and NAD-MDH in tender stems. Magnesium deficiency reduced the activity of PEPC, leading to a decrease in oxaloacetic acid content, limiting the process of CO₂ conversion to malate. The accumulation of malate at night is reduced, and the raw materials for photosynthesis are insufficient during the day, thereby reducing the accumulation capacity of organic matter. Conclusion: The optimal magnesium concentration for the cultivation of domestic red-fleshed pitaya is 2.0 mmol/L. The magnesium concentration in the soil of the current pitaya planting area is lower than this value, so it is recommended to reasonably increase the application of magnesium fertilizer during the planting of pitaya.