<p>Photosynthesis is the cornerstone of life on earth. Precisely evaluating the photosynthetic rate is critical to uncovering its underlying mechanisms. Numerous methods and tools have been developed to assess plant photosynthetic rate. However, these methods typically measure the carbon assimilation rate indirectly during photosynthesis, highlighting the need for a new method to determine plant photosynthetic rate precisely. This study developed an easy and reliable method using ¹³C isotope-labeled CO₂ and isotope element spectrometer technology to measure plant photosynthetic rate (C13MP). C13MP can precisely quantify the abundance of assimilated carbon by photosynthesis in plants. C13MP can also evaluate the carbon flow from leaves to the other organs by tracking the ¹³C isotope. The photosynthetic rates determined with C13MP were significantly correlated with maize biomass-related and yield-related phenotypes, suggesting the reliability of this method. In conclusion, C13MP can easily, precisely, and reliably evaluate carbon assimilation in various parts of the plant, providing a new method for measuring plant photosynthetic rate.</p>