The invention relates to a passive cooperative localization method for a radiation source target at sea according to observed values of double-station radars under the condition of the beyond visual range base line. Aiming at the problem that as under the influence from the earth curvature under the condition of the beyond visual range base line, the base line between the double-station radars is lengthened and angle measurement errors are introduced, the localization precision is poor, the passive cooperative localization method applicable to the beyond visual range base line is adopted to amend the radar measurement errors introduced by the earth curvature to achieve localization. The method comprises the specific steps that (1), coordinates of radar stations 1 and 2 are converted; (2), the detection range of a virtual target is generated initially; (3), the position offset of the virtual target relative to the station 1 is calculated; (4), the coordinates of the virtual target are calculated; (5), the observed value of the virtual target by the station 2 is calculated; (6), the measurement errors are judged; (7), the detection range of the virtual target is updated; (8), a localization result is output. By means of the method, the radar measurement errors introduced by the earth curvature can be amended, and the precision of the radar beyond visual range base line passive cooperative localization is improved effectively.