  The planets with a radius $<$ 4 $R$$_\oplus$ observed by the Kepler mission
exhibit a unique feature, and propose a challenge for current planetary
formation models. The tidal effect between a planet and its host star plays an
essential role in reconfiguring the final orbits of the short-period planets.
In this work, based on various initial Rayleigh distributions of the orbital
elements, the final semi-major axis distributions of the planets with a radius
$<$ 4 $R_\oplus$ after suffering tidal evolutions are investigated. Our
simulations have qualitatively revealed some statistical properties: the
semi-major axis and its peak value all increase with the increase of the
initial semi-major axis and eccentricity. For the case that the initial mean
semi-major axis is less than 0.1 au and the mean eccentricity is larger than
0.25, the results of numerical simulation are approximately consistent with the
observation. In addition, the effects of other parameters, such as the tidal
dissipation coefficient, stellar mass and planetary mass, etc., on the final
semi-major axis distribution after tidal evolution are all relatively small.
Based on the simulation results, we have tried to find some clues for the
formation mechanism of low-mass planets. We speculate that these low-mass
planets probably form in the far place of protoplanetary disk with a moderate
eccentricity via the type I migration, and it is also possible to form in situ.
