In this contribution, the preformation factors of 606 nuclei are extracted within the framework of the generalized liquid drop model (GLDM). Through the systematic analysis of the preformation factors of even-even Po-U isotopes, we found that there is a significant weakening of influence of shell closure in uranium, which is consistent with the results of a recent experiment [J. Khuyagbaatar et al., Phys. Rev. Lett. 115, 242502 (2015)], implying that may not be the magic number for U isotopes. Furthermore, we propose an improved formula with only 7 parameters to calculate preformation factors suitable for all types of -decay; it has fewer parameters than the original formula proposed by Zhang et al. [H. F. Zhang et al., Phys. Rev. C 80, 057301 (2009)] with higher precision. The standard deviation of the preformation factors calculated by our formula with extracted values for all 606 nuclei is 0.365 with a factor of 2.3, indicating that our improved formula can accurately reproduce the preformation factors. Encouraged by this, the -decay half-lives of actinide elements are predicted, which could be useful in future experiments. Notably, the predicted -decay half-lives of two new isotopes Np [Z. Y. Zhang, et al., Phys. Rev. Lett. 122, 192503 (2019)] and Np [H. B. Yang et al., Phys. Lett. B 777, 212 (2018)] are in good agreement with the experimental -decay half-lives.