An unified fission model (UFM) has been improved by considering the nuclear deformation effect and introducing an analytical expression of preformation factor. The improved version of the UFM by taking into consideration the nuclear deformation effect is named IMUFM1. Based on the IMUFM1, the further improved version is termed IMUFM2, which incorporates an analytical expression of the preformation factor. Within the UFM, the IMUFM1 and the IMUFM2, the α decay half-lives of heavy and superheavy nuclei with Z \geqslant 92 are systematically calculated. The calculated standard deviation between the calculation results and the experimental data shows that the accuracy of the IMUFM1 is improved by 2.45% compared with that of the UFM. The accuracy of the IMUFM2 will be further improved by 32.09% compared with that of the IMUFM1, which implies that the nuclear deformation effect and the preformation factor are both important in prediction. Then, the α decay half-lives of Z = 118–120 isotopes are predicted from the IMUFM1 and the IMUFM2 by inputting the α decay energy values that are extracted from the sinite-range droplet model (FRDM), the Weizsäcker-Skyrme-4 (WS4) model and the Koura-Tachibaba-Uno-Yamads (KTUY) formula, respectively. The observed evolution of the α decay half-lives indicates that the evolution trends obtained from the above-mentioned three mass models are consistent with each other and the shell effects occur at N = 178 and 184, but their orders of magnitude, obtained from different mass models, are different from each other. Meanwhile, the comparison of half-lives between α decay and spontaneous fission shows that the dominant decay modes of the superheavy nuclei with N < 186 are α decay. Finally, the decay modes of ²⁹⁶Og, ²⁹⁷119 and ²⁹⁸120 α decay chains are predicted within the IMUFM1 and the IMUFM2 by using these three mass models, showing that the predictions from the WS4 mass model and KTUY mass model are more consistent with the experimental measurements. Form the FRDM2012 mass model, the predictions of ²⁸⁸Fl, ²⁸⁵Nh and ²⁸⁶Fl within the IMUFM1 mass model are not consistent with the experimental measurements, however, the prediction of ²⁸⁸Fl from the IMUFM2 is good agreement with the experimental measurement, which once again verifies the rationality and reliability of the IMUFM2. This study may be helpful for identifying new nuclide in future experiments.