We assume that dark matter and dark energy satisfy the unified equation of state : p = B ( z ) \rho , with p = p _ { dE } , \rho = \rho _ { dm } + \rho _ { dE } , where the pressure of dark matter p _ { dm } = 0 has been taken into account .
A special function B = - \frac { A } { ( 1 + z ) ^ { \alpha } } is presented , which can well describe the evolution of the universe .
In this model , the universe will end up with a Big Rip .
By further simple analysis , we know other choices of the function B can also describe the universe but lead to a different doomsday .