1. A three-dimensional cardiac image segmentation method, comprising: searching from layers of cardiac computerized tomography CT images to obtain a layer of cardiac CT image located between a heart bottom portion and a heart upper portion, and taking the layer of cardiac CT image as a first key layer; searching on the first key layer to obtain a boundary of the heart, taking the boundary as a contour of the heart bottom portion, generating a curved surface model based on the contour of the heart bottom portion and a lowest point of the heart bottom portion, and taking the curved surface model as a heart bottom model; and searching layers of cardiac CT images above the first key layer to obtain boundaries of the heart, taking the boundaries as the contour of the heart upper portion, generating a curved surface model based on the contour of the heart upper portion, and taking the curved surface model as a heart upper model.