Pelvis plays a central role in regulating spinal sagittal balance and studies have highlighted the strong relationship between pelvic morphology and spinal alignment. However, correlation between these parameters were not well established in previous studies among adolescent idiopathic scoliosis (AIS) patients. These earlier studies were limited by small sample sizes, and none specifically focused on patients with major thoracic curves. The objective of this study was to analyse sagittal spinopelvic parameters in AIS patients with Lenke 1-4 curves and to investigate the correlations among thoracic kyphosis (TK), lumbar lordosis (LL) and pelvic incidence (PI). A retrospective study was conducted analysing preoperative sagittal radiographic parameters of 506 AIS patients (Lenke 1–4) who underwent posterior spinal fusion between 2017 and 2024. PI, global and segmental lumbar lordosis, segmental thoracic kyphosis and thoracolumbar junction angle were measured. Correlations among these parameters were evaluated. PI correlated positively with LL (r=0.395, p<0.001), primarily driven by proximal lumbar lordosis (PLL) (r=0.470, p<0.001), while distal lumbar lordosis showed no significant correlation. PI correlated negatively with TK (r= -0.199, p<0.001), predominantly due to distal thoracic kyphosis (DTK; r= -0.276, p<0.001) with no significant correlation observed between PI and proximal thoracic kyphosis (PTK) or middle thoracic kyphosis (MTK). PLL showed an inverse correlation with the thoracolumbar junction (r= -0.501, p<0.001), while the thoracolumbar junction correlated positively with DTK (r=0.700, p<0.001). A reciprocal relationship is observed between PI and TK, predominantly driven by inverse relationship between PI and DTK. This relationship appears to be mediated through the thoracolumbar junction, whereby increased PLL is associated with a less kyphotic thoracolumbar junction, which in turn is positively associated with DTK. PI significantly influences PLL while DLL remains relatively constant. These findings demonstrate coordinated segmental sagittal alignment between the lumbar and thoracic spine, with the thoracolumbar region representing an intermediate linkage within the overall spinopelvic profile. This study provides insight to guide surgical planning toward restoration of physiologic sagittal alignment in AIS patients.