Tagged MRI affords a non-invasive method for tracing the motion in myocardium. Some displacement information from the spare distorted tagging curves sampled in certain given times of the heartbeat period, can be obtained by the segmented result in tagged MRI. It is a challenge to recover nearly all the LV motions with the information. In this paper a model for calculating cardiac deformation is innovated. At first, the intersection points of tagging lines, which are obtained from the segmented result in tagged MRI, are used for fitting the local myocardial displacement with BPNN. Then the material points motion in heart are calculated by iterative computation of the displacement functions and the myocardial deformation is calculated. Experiments show that this model has advantages of explicit physical meaning, simple algorithm and high precision in calculation in the model.