Considering the fact that there is a correlation between the black hole horizon and cosmological horizon, we show that the thermodynamic entropy of de Sitter spacetime is the sum of thermodynamic entropies of the black hole horizon and cosmological horizon. The thermodynamic properties of de Sitter spacetime are also derived. It is shown that the upper limit energy of de Sitter spacetime is equal to the energy of pure de Sitter spacetime, The thermal capacity of de Sitter spacetime is negative, so de Sitter spacetime is mechanically unstable.