Traditional energy harvesters only work in a single frequency band and cannot make full use of the RF energy in the environment. This paper proposes an optimized multi-band RF energy harvesting system, which can efficiently work in different frequency bands. The structure of this harvester consists of a wideband receiving antenna linking multiple narrowband rectifier branches. Each branch is composed of a bandpass filter, an impedance matching network, and a multi-stage Dickson charge pump circuit. The output power of each branch is summarized by a diode summation network. On the basis of the existing multi-band RF energy harvester, we optimize the impedance matching network, the rectifier, and the diode summation network, which further improves the system performance. The results show that when the input power is 3 dBm, the power conversion efficiency of this system under the simulation conditions at 1 GHz reaches the maximum value of 78.3%. In addition, we also verify the possibility of this multi-band RF energy harvester to provide energy for backscatter communication in the 2.4 GHz ISM band. The simulation results show that the system can reach a power conversion efficiency of more than 50% in the three selected bands.