This is an open-label, single-arm, multi-center Phase II clinical trial evaluating the efficacy and safety of a novel sequential regimen as first-line therapy for treatment-naïve patients with Extranodal NK/T-cell Lymphoma (ENKTL). The study consists of a Screening Phase, a Safety Lead-in Phase, and a Treatment Phase. During the Safety Lead-in Phase, 6 patients will be enrolled to receive a fixed dose of Sintilimab and Chidamide combined with Azacitidine to verify the dose (testing 100mg/d on days 1-3 versus days 1-5). Following the lead-in, all subjects will undergo a 2-cycle Immunotherapy Induction Phase with the SCA regimen (Sintilimab, Chidamide, and Azacitidine). Subsequently, treatment will be stratified based on response: patients achieving Complete Response (CR) or Partial Response (PR) will receive 4 additional cycles of SCA consolidation, while those with Stable Disease (SD) or Progressive Disease (PD) will switch to 4 cycles of P-GemOx chemotherapy. Upon completion of systemic therapy, all patients will undergo consolidative involved-field radiotherapy (≥50Gy).