Vortex-induced vibration (VIV) is a typical type of wind-induced vibration of long-span bridges, which has the potential to cause large-amplitude oscillation at low wind speeds. The span-wise correlation of vortex-induced forces (VIFs) should be considered in the prediction of vortex-induced responses. At present, forced-motion tests with pressure measurements are often used to investigate the span-wise correlation of VIFs, but these tests may not be able to truly represent the nonlinear wind-structure interaction in VIV. This paper uses a newly-developed wind tunnel test technique to measure vortex-induced responses and forces at different span-wise locations. This study investigates the frequency-domain characteristics and the span-wise correlation of the VIFs on a twin-box deck under both smooth and turbulent flow fields. The results show that the fundamental and triple-frequency components of the VIFs play major roles in the VIV. The fundamental-frequency component of the VIFs can be seen as perfectly correlated within the lock-in range under smooth wind flow, even when the structural damping ratio increases from 0.35% to 0.8%. Under turbulent flow field, the correlation decays mildly with the increase of turbulence intensity, but within the lock-in range, the VIF is still well correlated even with a turbulence intensity of 8%.