Context : Synchrotron radio emission is being detected from an increasing number of galaxy clusters . Spectral index images are a powerful tool to investigate the origin , nature , and connection of these sources with the dynamical state of the cluster . Aims : The aim of this work is to investigate the spectral index distribution of the radio halo in the galaxy cluster A520 , a complex system from an optical , radio , and X-ray point of view . Methods : We present deep Very Large Array observations in total intensity at 325 and 1400 MHz . We produced and analyzed spectral index images of the radio halo in this frequency range at a resolution of 39 ^ { \prime \prime } and 60 ^ { \prime \prime } and looked for possible correlations with the thermal properties of the cluster . Results : We find an integrated radio halo spectral index \alpha _ { 325 } ^ { 1400 } \sim 1.12 . No strong radial steepening is present and the spectral index distribution is intrinsically complex with fluctuations only partially due to measurement errors . The radio halo integrated spectral index and the cluster temperature follow the global trend observed in other galaxy clusters although a strong point-to-point correlation between the spectral index and the thermal gas temperature has not been observed . Conclusions : The complex morphology in the spectral index image of the radio halo in A520 is in agreement with the primary models for radio halo formation . The flatness of the radial profile suggests that the merger is still ongoing and is uniformly and continuously ( re- ) accelerating the population of relativistic electrons responsible of the radio emission even at large ( \sim 1 Mpc ) distances from the cluster center .