Impacts of the total period number for the In_0.83Al_0.17As/In_0.52Al_0.48As digitally-graded metamorphic buffer (DGMB) on the performances of 2.6 μm In_0.83Ga_0.17As photodiodes (PDs) have been investigated. An increase of the total period number from 19 to 38 for the In_0.83Al_0.17As/In_0.52Al_0.48As DGMB with the same thickness has shown improved crystal qualities for the In_0.83Ga_0.17As/In_0.83Al_0.17As photodiode layers grown on such pseudo-substrates. An increased strain relaxation degree up to 99.8%, a reduced surface roughness, enhanced photoluminescence intensities as well as photo responsivities, and suppressed dark currents are observed simultaneously for the In_0.83Ga_0.17As photodiode on the DGMB with a period number of 38. These results suggest that with more periods, DGMB can restrain the transmission of the threading dislocations more efficiently and reduce the residual defect density.