At present, the most important problem of quantum key distribution is how to realize the key transmission for long distance and high key generation rate. Dual detectors are used in quantum key distribution and simulations are performed for BBM92 quantum key distribution system with ideal and parametric down conversion sources. Simulation results demonstrate improvements of keys generation rate in BBM92 protocol using dual detectors, as well as lower error bit rate. Secure communication distance can reach 300km when ideal sources are introduced and 250km with parametric down conversion sources.