to the controlling end of the plc 29 the plc 29 is equipped with a touch screen 30 a downward pressing bar 31 is additionally equipped in the third stage draft zone 3 in addition the drafting mechanism has a cradle 24 and a fiber strip feeding device 8 at the entrance it should be noted that the detectors 21 22 and 23 are all products of the prior art in the market they may be a detector which can simultaneously detect the twist and fiber weight of the target object or be the combination of a twist detector and a fiber weight detector the method for drafting spun yarns in three stages in the embodiment comprises the following steps step 1 a fiber strip 32 whose twist and weight are detected in advance is fed from the entrance of the drafting mechanism the fiber strip 32 is drafted in the first stage draft zone 1 to obtain a fiber strand 4 the twist of the fiber strand 4 is 75 to 95 of that of the fiber strip 32 and the weight of the fiber strand 4 is 71 to 99 of that of the fiber strip 32 the draft ratio in the first stage draft zone 1 is 1 01 to 1 40 in step 1 the detector 21 in the first stage draft zone 1 sends a detection signal containing the twist and fiber weight of the obtained fiber strand 4 to the controller 29 the controller 29 compares the detection signal with the preset value or scope of preset value of the first stage draft zone 1 and adjusts the revolutions of the rear drafting roller pair and the middle rear drafting roller pair through the corresponding servo motors 25.