In order to maximize the power spectrum utilization (PSU) while satisfying the Federal Communications Commission (FCC) emission mask, an improved two-stage ultra-wideband (UWB) pulse shaping method based on Gaussian derivatives is presented. Firstly, sub-pulses are designed by optimal combination of Gaussian derivatives over specific sub bands with the condition of maximizing the PSU allowed by emission mask. Secondly, the full band UWB pulse is shaped by the combination of the outputs of parallel band-pass filters (BPFs) bank with the sub pulses as inputs to these BPFs. Simulation results show that the UWB pulse shaped not only meets the FCC emission mask in full band, but also uses the limited power capacity more effectively, improving the PSU rate to enhance the performance of received UWB signal processing possibly. The proposed idea of sub band pulse design may be useful in narrow band interference mitigation as well in application of UWB technology.