To better understand the formation and decay of sunspot penumbrae, we studied the evolution of sunspots in three regions of the active region NOAA 12673 in detail. The evolution of sunspots in the three regions was involved in the interaction of two magnetic field systems: the preexisting magnetic field system and the later-emerging magnetic field system. Through analyzing the photospheric magnetic field properties, it is found that the formation of the penumbra originated from newly emerging magnetic bipoles that were trapped in the photosphere. The change in magnetic field in a penumbra from horizontal to vertical can cause the disappearance of the penumbra. A transformation of the magnetic field between the umbra and the penumbra is found, and the outward moat flow around the sunspot gradually decreased and vanished during decay of the sunspot. In addition, we found that the mean longitudinal magnetic strength in the penumbra decreased and the mean transverse magnetic strength in the penumbra increased with the increasing penumbral area during the formation of sunspots. However, during the decay of sunspots, the mean longitudinal magnetic strength in the penumbra increased, and the mean transverse magnetic strength in the penumbra decreased with decreasing penumbral area. Comparatively, the dependence of the area and the mean transverse/longitudinal magnetic field strength in the umbra is not remarkable. These results reveal that the formation and decay process of umbra are different from penumbra.