Formation of nitrogen-vacancy (NV) centers and their apparent charge-state response in high-pressure high-temperature (HPHT) type-Ⅰb diamond were investigated. Effects of the initial substitutional nitrogen content (C-center nitrogen), electron irradiation, vacuum annealing, and HPHT pretreatment on the photoluminescence (PL) behavior of NV centers were compared. Two groups of HPHT diamond single crystals with different C-center nitrogen contents were subjected to electron irradiation, vacuum annealing (600−1000 ℃), and HPHT pretreatment (5 GPa, 1100−1900 ℃). Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and photoluminescence spectroscopy were employed to analyze nitrogen aggregation, lattice state, and NV-related photoluminescence responses. The results show that high-nitrogen samples exhibit more pronounced NV-related emission, whereas irradiation-induced vacancies in the low-nitrogen samples tend to remain as GR1 centers. As the vacuum annealing temperature increases, the overall emission intensities of NV⁰ and NV⁻ increase, while the apparent photoluminescence response of NV⁻ relative to NV⁰ generally decreases. HPHT pretreatment does not directly generate a large number of NV centers but promotes nitrogen aggregation and modifies the apparent NV⁻/NV⁰ charge-state ratio during subsequent irradiation and annealing. These results indicate that HPHT pretreatment can serve as a preceding processing parameter for regulating the initial defect state before irradiation and the subsequent apparent charge-state ratio of NV centers.