The study of the paper concerns with the timing and synchronization problems in the impulse radio-ultra wideband (IR-UWB) positioning system. A novel synchronization algorithm based on two-stage enveloping method to improve the conventional sliding correlation performance is proposed. The correlator output is processed by two-stage enveloping firstly, then the fine synchronization interval is determined by centering the highest point in the second stage envelope. Time delay estimation (TDE) is completed by peak-picking of the absolute value at the correlator output in fine synchronization interval. The simulation results show that the proposed algorithm has a significantly better synchronization performance when the number of integrators is reduced, which make it very practical to use.