The weight measurement instrument includes a support frame, a plant holder, and a load cell. The plant holder is assembled with the support frame and has an upper opening and a lower opening, a hydroponic plant can be planted in the plant holder and grow towards the upper opening and the lower opening the load cell is fixed with the support frame and is used for measuring the total weight of the hydroponic plant automatically. A method for measuring the total weight of the hydroponic plant automatically is also provided.
