A beamformer method based on multiple time windows is proposed to estimate the equivalent current sources of different strength and duration from multi-channel magnetocardiography data. This technique of visualizing cardiac electric activities will be a great help to the diagnosis of relevant heart diseases. Feasibility of this beamformer method has been proved by simulation experiments of two known sources. Source estimation results and two-dimensional excitation propagation diagrams of a complete left bundle branch block and a complete right bundle branch block patients during QRS complex are taken to be compared. The revealed characteristics of their electrophysiological phenomena are analyzed.