Aiming at the existing problems in the applications of traditional track initiation approaches for bearing-only target tracking, a fast track initiation approach for single passive sensor system is proposed. In this approach, the rule of screening out threatening target is set through analysis of the target motion characteristics in polar coordinates. And the track initiation of threatening target is implemented in the shortest time based on a modified heuristic approach. Then, other target tracks are initialized based on a logic-based approach in polar coordinates. The cooperation of the two approaches makes track initiation more effective which provides a base for determining the threatening level of target in bearing-only target tracking system. The application of the proposed approach in a simulation and engineering system indicates its effectiveness and practicability.