Recent observations show that a large number of Lya emitters at high redshift z > 3 have unusually high Lya equivalent widths (EW > 400 Å). However, the origin of these high EWs is an open question. Here, we investigate the impacts of photon trapping on the Lya EW and other properties by tracking the Lyα radiative transfer in spherical galactic clouds. We find that the delayed escape of the Lya photons can change the Lya properties significantly. During the transition phase from optically thick to optically thin where the Lyα photons can escape simultaneously, the EW can be boosted to ~1000 Å, the Lyα luminosity can be increased by a factor of a few, and the line profile can be significantly broadened. The boost factor appears to depend on the galaxy properties such as mass and star formation rate and time-scale; therefore, future investigation combing 3D Lya radiative transfer calculations with cosmological simulations of galaxy formation and evolution is needed to fully understand the Lya properties of early star-forming galaxies. .
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