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The existing time-of-arrival-based UWB localization methods are introduced,which include time-of-arrival-based method, time-sum-of-arrival-based and timedifference-of-arrival-based method. The advantages of standard TDOA method related to the other methods are pointed out.Aiming at the difficulty of synchronization for standard TDOA method, an improved TDOA method is proposed, Cramer-Rao lower bound (CRLB) of this method is derived as performance benchmark. Results show that there is 2dB loss in performance compared to standard TDOA method, however, no synchronization requirement exists in the improved method, the complexity of the system is greatly reduced.The performance of the improved-TDOA-based localization is optimized, the impact of clock drift and anchor node configuration on the proposed method is analyzed, the factors that affect the performance of UWB positioning system is summarized through formula derivation and simulation results, anchor node configuration optimization is analyzed too. Results show that there exists an radius which optimizes the localization performance, and closed- form expression for the optimal radius and localization area radius is given, which can be used as a reference in practical UWB positioning system design.