The invention discloses a grass trimmer having the semi-balance deflection angle and provided with a parabolic wing blade LPG (liquefied petroleum gas) engine and solves the problem of axial movement for the original taper of a magnetic flywheel. According to an equation for arcs of blades, the bending angle is 18 degrees, the chord length b is 20, the ratio of a to b is 0.45, and a refers to the maximum deflection point on the bow; an intermediate technician can make a mold module for the wing section contour line. Outer ends of the blades are sharp ends and have cutting edges, so that entangled materials can be cut off conveniently. The balance deflection angle (phi) exists between an angle bisector of the central angle from two sides of an opening, where a C-shaped ring (56a) of a crankshaft valve (56) is mounted, of a crankshaft (57) to a circle center of the crankshaft and a central connecting line from the circle center of the crankshaft to a large-end circle center of a connecting rod and is in the range of 2-22 degrees, and the single-cylinder semi-balance is improved. A cutter is provided with an outer protecting cover (73) mounted on an inner protecting cover (60), and a long grass pressing wheel (74) with the roller length equal to the grass trimming diameter is arranged on the outer protecting cover of the cutter, so that grass pressing water retention and grassland pattern pressing are facilitated; multiple sharp edges (54a) are arranged on a buttress cutting and grass trimming steel wire (54b) of a grass trimming head (54) of the cutter and cut out on the steel wire by a sharp flat-ended chisel under hammering.