An open plant cultivation device has a container, a medium layer, a plant, and a separation layer. The container has an inner side surface, a sterile accommodation space, and an open growing space. The medium layer is disposed in the sterile accommodation space. The separation layer is disposed on the medium layer and adjacent to the inner side surface. The sterile accommodation space and the open growing space are isolated by the separation layer. A method for preparing an open plant cultivation device includes: putting a container in a sterile space feeding a medium colloid into the container and immersing a part of a plant in the medium colloid solidifying the medium into a medium layer disposing a separation layer onto the medium layer to form the open plant cultivation device. The present invention has the open growing space and the effect of protecting the medium layer from contamination.
