The invention relates to an airplane flexible control system, in particular to a steel rope pretension force design method of the airplane flexible control system. The steel rope pretension force design method comprises the following steps: (1) calculating the influence of temperature on an airframe loop; (2) calculating a steel rope loop variation caused by temperature change when a steel rope is installed on an airplane; (3) calculating pretension forces required to be set when the steel rope is at the minimum temperature and the normal temperature; (4) generating a ground surface steel rope pretension force setting curve when the temperatures are different; (5) generating a ground surface steel rope pretension force setting curve when the temperatures are different; (6) setting the steel rope pretension force according to the setting law; and (7) performing flight verification on the setting law. Compared with the prior art, the steel rope pretension force design method has the beneficial effects that the problems that the system friction force is increased and the airplane controllability and the service life of the steel rope are influenced since the steel rope pretension force is too small when the flexible system works or too large after idle stroke and control delay appear when the system works are solved, and the steel rope pretension force design blank of domestic medium-sized airplane flexible control systems is filled.