Risk factors of proximal junctional kyphosis (PJK) remain a topic of debate for and no consensus regarding the optimal UIV to prevent this complication has been proposed. While many authors suggest avoiding to stop the fusion at the thoracolumbar area, other studies concluded the location of UIV does not have any clear effect on the development of PJK. We conducted a prospective study from January 2020 to April 2025, recruiting adult spinal deformity (ASD) patients who underwent from 4 level of TLIF as surgical treatment. Participants will be allocated into two groups: UIV at the thoracolumbar area (TL group) and UIV outside of the thoracolumbar area (noTL group). PJK rate in both groups has been calculated and analyzed in order to assess the relationship between the location of UIV and PJK. There were 72 patients (12 men/60 women) with mean age of 64 and mean FU duration of 2.5 years. The PJK rate of the whole group was 15% (11/72), meanwhile PJK rates of the TL and noTL groups were 24% (6/25) and 10% (5/47), respectively. There was no statistically significant relationship between the location of UIV and the risk of PJK at final FU (OR=2.25, p=0.22). Among other factors (age, gender, length of fusion, postop sagittal imbalance and amount of sagittal correction), only sagittal correction more than 30o is proven to be the risk factor of PJK (OR=20, p<0.001). Our result was in line with recent studies when demonstrating that stopping the fusion at the thoracolumbar area does not increase the risk of PJK afterward. This finding advocates the tendency of minimizing the fusion length which in turn can reduce the aggressiveness and related complications of correcting surgery in elderly ASD patients.