The invention provides a method for detecting a baseline margin of a sewing machine. The method successively comprises the steps that a residual baseline wound on a wrapping pillar drives a bobbin to rotate, and the bobbin drives multiple sensed parts on a flange to rotate synchronously; a sensor senses the sensed part exposed from a through hole of a bobbin sleeve and then outputs a pulse signal, wherein the pulse signal is formed by high levels and low levels which are distributed at intervals; an electric control module of the sewing machine receives the pulse signal from the sensor and detects a level frequency of the pulse signal; when the level frequency of the pulse signal increases, the electric control module can determine that the baseline margin on the wrapping pillar is enough and can control a main motor of the sewing machine to continue rotating; and when the level frequency of the pulse signal decreases, the electric control module determines that the baseline margin on the wrapping pillar is insufficient and controls the main motor of the sewing machine to stop rotation. According to the application, whether the baseline margin on the bobbin is enough is determined according to rotation speed changes of the bobbin. The method has the advantages of very high detection accuracy and high reliability.