The invention relates to a method for segmenting a cardiac nuclear magnetic resonance image. The method for segmenting the cardiac nuclear magnetic resonance image comprises the following steps of: 1, performing Gaussian filtering preprocessing on the acquired cardiac nuclear magnetic resonance image; 2, computing an external force field of a generalized gradient vector flow based on expansion neighborhood and noise smoothing on the preprocessed image; 3, defining an initialized outline position of the left ventricular endocardium of the heart; 4, segmenting the left ventricular endocardium of the heart; 5, defining a final segmenting outline result of the left ventricular endocardium of the heart into an initialized outline position of the left ventricular epicardium of the heart; 6, setting the boundary strength of an area surrounded by the endocardium outline in an original boundary graph to be 0, recomputing the external force field of the generalized gradient vector flow based onthe expansion neighborhood and the noise smoothing; and 7, segmenting the left ventricular epicardium of the heart. Based on convolution computation, and by taking energy constraint of an elliptical shape, the method for segmenting the cardiac nuclear magnetic resonance image has the advantages of high computing speed, wide capturing range, strong anti-noise ability, excellent performance in weakboundary protection and deep dented region segmentation, and capability of accurately segmenting the left ventricular endocardium and the left ventricular epicardium of the heart.