Purpose To compare structural and vascular parameters in the central and peripheral retina and choroid of eyes diagnosed with severe non-proliferative diabetic retinopathy (NPDR) or proliferative diabetic retinopathy (PDR) with or without pan-retinal photocoagulation (PRP) using ultrawide-field wept-source optical coherence tomography angiography (SS-OCTA). Methods All participants underwent SS-OCTA imaging centered on the fovea. Retinal and choroidal thickness, vessel density of the superficial capillary plexus (SCP), deep capillary plexus (DCP), choroidal capillary plexus (CCP), and choroidal Sattler’s and Haller’s layer (CSHL) were analyzed in nine grids using built-in angiography analysis software. Results A total of 82 eyes from 82 participants were enrolled in this study, including 40 eyes diagnosed with severe NPDR or PDR without PRP and 42 eyes with PRP. Retinal thickness in part grids was higher in eyes with PRP than in eyes without PRP. Vessel density of the SCP in nasal-superior (P = 0.003) grid was lower in eyes with PRP than in eyes without PRP. No significant difference was found in the vessel density of the DCP between the two groups (all P > 0.05). The choroidal thickness and vessel density of the CSHL of all grids significantly lower in eyes with PRP than in eyes without PRP (all P < 0.05). A statistically significant decrease in vessel density in CCP was found in the superior (P = 0.043), nasal-superior grid (P = 0.003), macular grid (P < 0.001), and optic disc grid (P = 0.001) of eyes with PRP, compared to eyes without PRP. A significant decrease in the vessel density of CSHL was observed in all grids of PDR eyes with PRP compared to PDR eyes without PRP (all P < 0.05). Significant decrease in choroidal thickness was observed in most grids of PDR eyes with PRP, except for macular grid (P = 0.090) and optic disc grid (P = 0.057). Conclusion Structural and vascular parameters in the central and peripheral retinal and choroidal layers in eyes diagnosed with severe NPDR or PDR with or without PRP could be quantified using a ultrawide-field SS-OCTA. Eyes with PRP showed a significant decrease in choroidal thickness and vessel density of CCP and CSHL, compared with eyes without PRP. This trend was more obvious in eyes with PDR.