An air compressor comprising a piston which is actuated by a motor to move upward and downward in a cylinder; the piston of the air compressor includes an air stop sheet (7, 9) mounted on a top support plate (21, 81) thereof, and the air stop sheet (7, 9) has a bending section (71, 94), wherein the bending section (71, 94) has a positioning zone (72, 91) arranged on the top support plate (21, 81), and the bending section (71, 94) further has an acting zone (73, 93) located opposite to the positioning zone (72, 91) and configured to cover an air channel (23, 89) of the piston, wherein the bending section (71, 94) includes at least one collapsible guide line (711, 941, 942) formed thereon to match with an output power of the air compressor, wherein the at least one collapsible guide line (711, 941, 942) is a boundary axis of the acting zone (73, 93) and the positioning zone (72, 91) of the air stop sheet (7, 9) so that a top surface of the air stop sheet (7, 9) facing the cylinder forms an obtuse angle less than 180 degrees, and a back surface of the acting zone (73, 93) of the air stop sheet (7, 9) backing a top of the cylinder turns on relative to a plane of a top of the top support plate (21, 81) at an open angle θ, thus producing an air flowing space, characterized in that the bending section (71,94) comprises a first collapsible guide line and a second collapsible guide line formed between the positioning zone and the acting zone, the acting zone has a passing orifice, the top support plate has a hook configured to engage with the passing orifice, when the air compressor stops, the acting zone of the air stop sheet turns on relative to the plane of the top of the top support plate, and the passing orifice of the air stop sheet is stopped by of the hook, and the positioning zone has two spaced orifices (911, 912) that pass through two separated extensions (85, 86) of a plane of a top of the top support plate and are stopped by two spaced circular shoulders (87, 88). the piston of the air compressor includes an air stop sheet (7, 9) mounted on a top support plate (21, 81) thereof, and the air stop sheet (7, 9) has a bending section (71, 94), wherein the bending section (71, 94) has a positioning zone (72, 91) arranged on the top support plate (21, 81), and the bending section (71, 94) further has an acting zone (73, 93) located opposite to the positioning zone (72, 91) and configured to cover an air channel (23, 89) of the piston, wherein the bending section (71, 94) includes at least one collapsible guide line (711, 941, 942) formed thereon to match with an output power of the air compressor, wherein the at least one collapsible guide line (711, 941, 942) is a boundary axis of the acting zone (73, 93) and the positioning zone (72, 91) of the air stop sheet (7, 9) so that a top surface of the air stop sheet (7, 9) facing the cylinder forms an obtuse angle less than 180 degrees, and a back surface of the acting zone (73, 93) of the air stop sheet (7, 9) backing a top of the cylinder turns on relative to a plane of a top of the top support plate (21, 81) at an open angle θ, thus producing an air flowing space, characterized in that the bending section (71,94) comprises a first collapsible guide line and a second collapsible guide line formed between the positioning zone and the acting zone, the acting zone has a passing orifice, the top support plate has a hook configured to engage with the passing orifice, when the air compressor stops, the acting zone of the air stop sheet turns on relative to the plane of the top of the top support plate, and the passing orifice of the air stop sheet is stopped by of the hook, and the positioning zone has two spaced orifices (911, 912) that pass through two separated extensions (85, 86) of a plane of a top of the top support plate and are stopped by two spaced circular shoulders (87, 88).