In the field transplanting process of vegetable plug seedlings, the lack of seedlings in the plug trays and the failure of picking and throwing seedlings lead to the leakage planting, resulting in waste of resources such as land, water and pesticides. In order to solve the problem of leakage planting, a plug seedling detection and seedling leakage supply system (seedling supply system) for a fully automatic transplanter of vegetable plug seedlings was designed. As an independent functional module, the seedling supply system specifically included a plug seedling detection unit and a seedling supply device. A reflective optical fiber sensor (OFS) and a laser sensor were used to detect the separation seedling cup and identify the plug seedling in the separation seedling cup. Pepper, tomato, cabbage plug seedlings were tested at a variety of detection height by OFS to obtain the best detection height and threshold setting parameters for OFS detection. With the height of the OFS probe was increased, the difference values was increased first and then decreased. When the feedback difference values were the largest, the corresponding plug seedling detection height was the best, and the optimal threshold parameter was set according to the feedback value and the feedback difference value. The automatic supply device was designed, and the movement analysis of the seedling supply process was carried out. The HMI-PLC control system was designed to realize the control of the transplanter and the seedling supply system. A field comparison experiment with different transplanting frequencies was carried out on the opening and closing of the seedling supply system. The experiment results showed that when the single row transplanting frequency was 60 plants/min, 70 plants/min, and 80 plants/min, the recognition success rate was 98.15 %, 98.15% and 97.69%, the leakage planting rate was 1.85%, 2.31% and 2.31%, respectively. Compared with the transplanting when the seedling supply system was not enabled, the leakage planting rate was reduced by 14.59, 14.36 and 15.52 percentage points, respectively.