The collecting line in wind farms is accessed in a dense fashion by complex power sources. This makes existing fault localization methods inapplicable. Therefore a fault localization method for asymmetric faults is proposed based on bi-level negative-sequence difference and negative-sequence fault distance calculation. First, the fault current characteristics of wind turbines (WTs) are analyzed, based on which mathematical models of WTs can be constructed from the perspective of negative-sequence. The equivalence of wind farm negative-sequence networks is further implemented considering the structural characteristics of wind farms. Then, introducing a fault simulation strategy, the difference between simulated and actual faults is characterized by the negative-sequence voltage difference to identify the first node of faulty areas. The variation trend of negative-sequence voltage from actual fault scenarios to specific simulated fault scenarios is explored, and the negative-sequence voltage ratio is defined to eliminate the effect of fault point current. From this a criterion based on the negative-sequence voltage ratio difference is proposed to achieve identification of the second node of faulty areas. Finally, fault locations are determined by a fault distance calculation formula derived from the two-terminal negative-sequence quantity of faulty areas. Verification results from PSCAD/EMTDC show that the proposed method can accurately locate various asymmetric faults of collecting lines by just a few measurement points. The applicability to collecting lines with line branches is also considered. The localization performance is not affected by fault locations, fault resistance, WTs, wind speed distribution or asynchronous data.