A method for reconstructing target cardiac images is provided. The method may include: obtaining projection data, the projection data including a plurality of sub-sets of projection data, each sub-set of projection data corresponding to a cardiac motion phase; obtaining a plurality of sampled cardiac motion phases; generating a plurality of cardiac images of the plurality of sampled cardiac motion phases by reconstructing, based on the one or more sub-sets of projection data corresponding to the each sampled cardiac motion phase, one or more cardiac images of the each sampled cardiac motion phase; determining a plurality of cardiac motion parameters corresponding to the plurality of sampled cardiac motion phases based on the plurality of cardiac images; determining a mean phase based on the plurality of cardiac motion parameters corresponding to the plurality of sampled cardiac motion phases; and reconstructing the one or more target cardiac images of the mean phase.