Abstract: The present disclosure discloses a method for preparing a labelled plant and a method for preparing a labelled biochar. The method for preparing a labelled plant includes the following steps: preparing a box; placing a plant that is capable of photosynthesis in the box; preparing a labelled gas in the box, the labelled gas being an isotope-labelled carbon dioxide gas; and leaving the box standing for a preset period of time, during which the plant absorbs the labelled gas to obtain a labelled plant. In the present disclosure, the carbon element in the plant is labelled during the growth of the plant, and the concentration of 13CO2 in the box is controlled by controlling the concentration of Na213CO3, where 13CO2 is prepared by chemical methods, which could improve the utilization rate of 13C.