For ultra-wideband positioning using impulse radio, intersymbol interference (ISI) is unavoidable, which affects the positioning accuracy. In this paper, a novel ISI cancellation scheme based on wavelet transformation for impulse positioning is proposed. Analog-to-digital conversion of the ultra-wideband signal is not needed, and knowledge of the channel is not required. The desired signal pulse can be distinguished from the interference pulse and precisely located. Instead of using the traditional received pulse, wavelet transformation is applied to generate a new spike signal for detection. A detailed theoretical analysis is presented and closed-form expressions are derived. Simulation results show that the proposed positioning scheme can cancel ISI effectively and achieve significantly lower average positioning error than the original TDOA system without ISI cancellation.