The invention discloses a method for detecting a transverse swinging angle of a vertically suspended flat elevator cable. The method comprises the following steps: fixing one end of a test sample cable, fixing the other end of the test sample cable on a trolley which can be moved up and down vertically, suspending into a U shape, respectively fixing naked copper hoses on two sides of the bottom ofthe U shape, respectively suspending auxiliary steel wire ropes in front of and behind the test sample cable, and connecting the naked copper hoses and the two auxiliary steel wire ropes with a single pulse trigger through insulating leads; in testing, moving the trolley of the test sample cable downwards till the cable is J-shaped, further moving the trolley upwards till the cable is U-shaped, and detecting the maximum transverse swinging angle of the bottom of the U shape of the test sample cable by moving the auxiliary steel wire ropes. By adopting the method disclosed by the invention, quality indexes which are directly related to dynamic stability of a cable and quantitative detection are defined, whether the dynamic stability of a flat elevator following cable which swings front andback meets design and use requirements or not can be also directly quantitatively detected, compounding matching of two flat cables can be conveniently achieved, and security reliability of elevatoroperation can be ensured.