The invention relates to crop growth detecting technology and discloses a detecting device and a detecting method for crop biomass. The detecting device comprises a bearing container, a power module, a plant height gathering module, a stem diameter gathering module, a clamping module and a data acquisition and analysis module, wherein the power module, the plant height gathering module, the stem diameter gathering module, the clamping module and the data acquisition and analysis module are respectively arranged on the bearing container. The power module supplies power for the whole device, the clamping module is used for clamping a sample to be detected, and the stem diameter gathering module and the plant height gathering module are respectively used for collecting stem diameter information and plant height information of the clamped sample to be detected and transmitting the information to the data acquisition and analysis module which conducts data processing and analysis and then building a theoretical and biomass prediction model. The plant height gathering module adopts a laser distance measuring sensor, and the stem diameter gathering module adopts a displacement sensor. The detecting device is flexible and portable, and can conduct rapid, free-damage and accurate measurement on crops such as naturally-grown wheat and rice with a plurality of stem on a single stem.