Hawking radiation of a Reissner-Nordstrom-de Sitter black hole

Generalizing the method proposed by Damour-Ruffini, we discuss Hawking radiation of a Reissner-Nordstrom-de Sitter (RNdS) black hole. Under the condition that total energy and charge are conserved, taking the reaction of the radiation of particles to the spacetime into consideration and considering the interrelation between the event horizon and cosmological horizon, we investigate radiation spectrum of RNdS spacetime by a new Tortoise coordinate transformation. This radiation spectrum is no longer a purely thermal spectrum. It is related to the changes in the Bekenstein-Hawking entropy corresponding the event horizon and cosmological horizon. The result satisfies the unitary principle.