The growth of Achnatherum splendens (A. splendens) inhibits the growth of dominant grassland herbaceous species, resulting in a loss of grassland biomass and a worsening of the grassland ecological environment. Therefore, it is crucial to identify the dynamic development of A. splendens adequately. This study intended to offer a transformer-based A. splendens detection model named YOLO-Sp through ground-based visible spectrum proximal sensing images. YOLO-Sp achieved 98.4% and 95.4% AP values in object detection and image segmentation for A. splendens, respectively, outperforming previous SOTA algorithms. The research indicated that Transformer had great potential for monitoring A. splendens. Under identical training settings, the AP value of YOLO-Sp was greater by more than 5% than that of YOLOv5. The model’s average accuracy was 98.6% in trials conducted at genuine test sites. The experiment revealed that factors such as the amount of light, the degree of grass growth, and the camera resolution would affect the detection accuracy. This study could contribute to the monitoring and assessing grass plant biomass in grasslands.