Reconstructing cardiac current source by magnetocardiogram signal is a noninvasive method of revealing cardiac electrical activity. The visualization of spatial and temporal information about cardiac electrical activity will be conducive to the analysis of the heart function and the diagnosis of heart disease. In this paper, the capability of reconstructing distribution of current sources, whose distribution is changed over time, is studied by a reduced time window (RTW) beamformer, and the relationship between source reconstruction results and ventricular excitation propagation is analyzed in simulation. The cellular automaton is used to simulate ventricular excitation propagation and to generate the current sources. Moreover, a heart-torso model is constructed by the boundary element method to simulate the effect of the volume conductor. The simulation results show that the RTW beamformer can provide a good accuracy of reconstructing the distribution of current sources. The RTW beamformer is also used to analyze a normal subject with the lead field of infinite homogeneous medium. The results demonstrate that they reflect the basic characteristic of ventricular excitation propagation.