The invention relates to a biological nutritional solution used for improving the plump rate of lotus seed pots and specially used for lotus flowers, and a preparation method thereof. The biological nutritional solution comprises, by weight, 10-25 parts of stems of Polygonum persicaria, 40-60 parts of leaves of Parnassia palustris, 3-12 parts of flowers of Epilobium hirsutum and 400 parts of water. The preparation method of the biological nutritional solution comprises the following steps: 1, juicing fresh stems of Polygonum persicaria, leaves of Parnassia palustris and flowers of Epilobium hirsutum, and filtering off solid impurities to leave clarified juices for later use; 2, uniformly mixing the above obtained three plant juices with 400 parts of water to obtain a mixed liquid for later use; and 3, injecting the mixed liquid to planted lotus stems through a syringe. The present situation of low plump rate of present lotus seed pots is solved, a new approach is provided for meeting market demands, and a new idea is provided for high utilization of Polygonum persicaria, Parnassia palustris and Epilobium hirsutum.