The crystal and electronic structure of antiferroelectric squaric acid is studied using density functional theory method,and the exchange correlation effects are treated by the generalized approximation. In order to understand the ferroelectricity of H2SQ in the molecular plane and the antiferroelectricity in whole crystal, the density of states,charge density distribution and band structure are calculated. The result showed that O2p and C2p play important roles in the interactions between layers. The hybridizations of O2s-Hls and O2p-Hls are responsible for the tendency to ferroelectricity within each layer.