Abstract: Triple-observer source localization using frequency difference of arrival (FDOA) measurements is an important passive localization method. One of the typical application scenarios is triple-aircraft passive localization of the fixed emitter. In order to support engineering effectively, it is necessary to assess the performance of triple-aircraft passive localization. Therefore, the localization models are firstly established, including the FDOA observation equations, navigation observation equations and earth surface constraint condition. And then, with the bias of FDOA and that of range between the emitter and the earth 's core (REEC), the expression of the mean square error (MSE) of the emitter’s position is deduced by perturbation method according to weighted least squares (WLS) principle. The expression shows the relationship between MSE and relative factors, such as the velocity of the aircrafts, the frequency of the emitter, the error of FDOA, the error of navigation parameters, the bias of REEC and so on. Finally, with the typical simulation parameters, the influence of multiple factors on the localization performance is analyzed by simulation. Several conclusions are obtained and their possible practical values are indicated in the paper.