As the economy develops and urbanization progresses, the amount of arable land continues to decline. In this context, the cultivation of double-season rice is particularly important for enhancing yield per unit area. However, research on the physiological mechanisms that contribute to high yields in double-season early rice varieties with short growing seasons is still limited. To address this gap, we conducted a field study using two early rice varieties, Zhongzu18 and Yongxian15, to examine their production characteristics, photosynthesis, fluorescence, and energy metabolism. The results indicate that Zhongzu18 has a significantly higher seed-setting rate, grain weight, and total grain yield compared to Yongxian15. Additionally, Zhongzu18 exhibits a higher head rice rate and a lower degree of chalkiness, along with a reduced chalky grain rate. Furthermore, the total dry matter weight and the ratio of panicle weight to total weight for Zhongzu18 were significantly greater than those for Yongxian15. After anthesis, Zhongzu18 also demonstrated a higher leaf net photosynthetic rate and actual fluorescence quantum efficiency compared to Yongxian15. Moreover, the levels of ATP and ATPase, as well as the activities of antioxidant enzymes and the expression of sucrose transport-related genes, were significantly increased in Zhongzu18 plants relative to Yongxian15. We conclude that the enhanced photosynthetic efficiency and energy production in Zhongzu18 lead to more effective assimilation and carbohydrate transport to the grains, resulting in higher grain yields and improved rice quality.