In this paper, a downlink cell-free Massive MIMO system (CF-M-MIMO-S) is considered. The CF-M-MIMO-S is a distributed M-MIMO-S, where access points (APs) with very great quantity of antennas, and a much smaller number of independent users are distributed randomly. Firstly, an approximate expression for the capacity with perfect channel state information and conjugate beamforming scheme is derived. Secondly, an energy-efficient (EE) resource allocation strategy is advanced, which is aim to maximize system EE. Specifically, the power consumption include transmitting power, calculation power and circuit power. Simulation results indicate that the throughput of derived approximate expression is very close to theoretical value. It is also demonstrated The effectiveness of proposed algorithm and the trade-off between EE and the quantity of Aps is, meanwhile, the performance of throughput of the proposed algorithm is very well.