A high performance packet detector for MB-OFDM UWB(Multiband Orthogonal Frequency- Division Multiplexing Ultra Wideband) system is proposed according to MB-OFDM UWB physical international standard ECMA-368. On the basis of conventional cross correlation，the detector can complete packet detection and fine symbol timing at the same time by accumulating multi-path energy. Simulation results show that the new detector has a lower packet detect error rate and a higher symbol timing precision compared with the existing schemes.