In downlink Cloud Radio Access Network(C-RANs), the data is transferred from a Base Band Unit(BBU) in the core network to several remote radio heads via a set of edge routers over capacity-limited fronthaul links. The remote radio heads then send the received signals to their users via radio access links. Now, energy efficiency of C-RANs is a topic issue. Therefore, the general downlink of C-RANs is studied in order to maximize the network energy efficiency by jointly optimizing User Association(UA), Radio Remote Head(RRH) activation, data rate allocation and signal precoding. By employing the successive convex quadratic programming framework, an iterative algorithm is proposed to solve the challenging mixed-integer nonlinear problem formulation. Simulation results show that the proposed joint optimization approach significantly improves the energy efficiency.