The spanwise correlation of vortex-induced forces （VIFs） is necessary to accurately predict vortex-induced vibration （VIV） amplitude of three-dimensional flexible structures. In this paper， the properties of aerodynamic spanwise correlation in the lock-in region are studied via theoretic modelling of aerodynamic spanwise correlation function and wind tunnel tests of VIV for the 5︰1 rectangular sectional model. The results show that in the vortex interval， the measured aerodynamic force consists of fully correlated self-excited force and incompletely correlated random aerodynamic force. The spanwise correlation can be described by exponential function and a constant， the exponential coefficient denotes the energy ratio of random aerodynamic force to measured aerodynamic force， and the sum of the exponential coefficient and the constant is equal to 1. The root mean square of measured aerodynamic lift is tend to be the stronger in the onset stage of VIV， but decreases with the increase of wind speed. The root mean square of random aerodynamic force seem to be identical. Therefore， the spanwise correlation of measured aerodynamic force in the onset stage is tend to the stronger than that in the following stage.