A grass trimmer, comprising: a trimmer head (20, 30, 40, 50, 60) comprising a spool (12, 38, 47, 52) and a head housing (111, 39, 46, 51) (111, 39, 46, 51), wherein the spool (12, 38, 47, 52) is configured for a trimmer line winding around, and the head housing (111, 39, 46, 51) is configured to accommodate at least part of the spool (12, 38, 47, 52); and a driving device configured to drive the trimmer head (20, 30, 40, 50, 60) to rotate around an axis, wherein the driving device comprises a motor having an output shaft, and the grass trimmer has an automatic winding mode in which the motor is configured to drive at least one of the spool (12, 38, 47, 52) or the head housing (111, 39, 46, 51) to cause a relative rotation between the spool (12, 38, 47, 52) and the head housing (111, 39, 46, 51) so as to wind the trimmer line onto the spool (12, 38, 47, 52); an operating assembly (17, 22) comprising an operating portion (172a, 222d) operable by a user; a winding switch (18, 21, 31, 45, 53) configured to control the automatic winding mode to be enabled when the operating portion (172a, 222d) is operated; and a locking mechanism (19, 23, 32, 43, 54), wherein the operating assembly (17,22) is configured to, when the operating portion (172a, 222d) is operated, trigger the winding switch (18, 21, 31, 45, 53) and cause the locking mechanism (19, 23, 32, 43, 54) to lock at least one of the head housing (111, 39, 46, 51) or the spool (12, 38, 47, 52) so as to cause the relative rotation between the spool (12, 38, 47, 52) and the head housing (111, 39, 46, 51), characterized in that the head housing (111, 39, 46, 51) is capable of rotating clockwise about the axis and also rotating counterclockwise about the axis when the operating portion (172a, 222d) is not operated. a trimmer head (20, 30, 40, 50, 60) comprising a spool (12, 38, 47, 52) and a head housing (111, 39, 46, 51) (111, 39, 46, 51), wherein the spool (12, 38, 47, 52) is configured for a trimmer line winding around, and the head housing (111, 39, 46, 51) is configured to accommodate at least part of the spool (12, 38, 47, 52); and a driving device configured to drive the trimmer head (20, 30, 40, 50, 60) to rotate around an axis, wherein the driving device comprises a motor having an output shaft, and the grass trimmer has an automatic winding mode in which the motor is configured to drive at least one of the spool (12, 38, 47, 52) or the head housing (111, 39, 46, 51) to cause a relative rotation between the spool (12, 38, 47, 52) and the head housing (111, 39, 46, 51) so as to wind the trimmer line onto the spool (12, 38, 47, 52); an operating assembly (17, 22) comprising an operating portion (172a, 222d) operable by a user; a winding switch (18, 21, 31, 45, 53) configured to control the automatic winding mode to be enabled when the operating portion (172a, 222d) is operated; and a locking mechanism (19, 23, 32, 43, 54), wherein the operating assembly (17,22) is configured to, when the operating portion (172a, 222d) is operated, trigger the winding switch (18, 21, 31, 45, 53) and cause the locking mechanism (19, 23, 32, 43, 54) to lock at least one of the head housing (111, 39, 46, 51) or the spool (12, 38, 47, 52) so as to cause the relative rotation between the spool (12, 38, 47, 52) and the head housing (111, 39, 46, 51), characterized in that the head housing (111, 39, 46, 51) is capable of rotating clockwise about the axis and also rotating counterclockwise about the axis when the operating portion (172a, 222d) is not operated.