We present here a systematic study on the mechanism of the long/short timescale variability and the origin of soft X-ray excess for narrow-line Seyfert 1 galaxies (NLSy1s) based on the multi-epoch multi-band observations of the Swift telescope. The main results are as follows: (1) The ensemble structure functions of radio-quiet NLSy1s in the optical, ultraviolet, and X-ray bands all show a unique double 'S' structure. The structural feature reveals that the long and short timescale variability of radio-quiet NLSy1s could be generated by the accretion fluctuation propagation mechanism and the X-ray reprocessing mechanism, respectively, which together cause the complex random light variability. (2) Radio-quiet NLSy1s exhibit significant soft X-ray excess features in their X-ray spectra, but the relative strength of the soft X-ray excess (R) and the power-law spectral index (Gamma) do not show a significant positive correlation, unlike what is seen in broad-line Seyfert 1 galaxies (BLSy1s), suggesting that NLSy1s and BLSy1 have different soft X-ray excess mechanisms. After combining the correlation analysis results of model parameters for individual sources at different flux states with the findings in previous work based on XMM-Newton and Suzaku data, we propose that the origin of the soft X-ray excess of NLSy1s prefers the relativistically smeared reflection scenario.