Due to the influence of ocean currents, Hadal landers deviate from the launch points during dives. The deviation offset and direction are difficult to be estimated. Moreover, the underwater acoustic positioning method is not suitable in the Hadal areas. Through the cross-line air-gun shooting operation carrying out on the sea surface, and by using the accurate position and timing information of each air-gun shot, as well as the arrival times of direct water waves recorded by the Ocean Bottom Seismometer (OBS), the seafloor positions of Hadal Landers can be measured precisely. In this method, the collected data are preprocessed and inversed for the actual landing positions of Wanquan Hadal Lander, using the Monte Carlo method combined with the least square method. The obtained position correction offsets of the three dives are 211 m, 178 m and 861 m respectively, with different deviation directions from each other. The measurement precision is +/- 20 m, which is much more accurate than the previous ones. In addition, this method determines precise locations of the three seafloor markers and provides reliable underwater reference coordinates for the future 10000 m-type unmanned or manned submersibles.