An air compressor comprising a piston which is actuated by a motor to move upward and downward in a cylinder; wherein the piston of the air compressor includes an air stop sheet mounted on a head thereof, and the air stop sheet has a bending section, wherein the bending section has a positioning zone arranged on the head, and the bending section further has an acting zone located opposite to the positioning zone and configured to cover an air channel of the piston; wherein a piston rod extends downward from the head, and the piston rod includes a cavity horizontally passing through a top of the piston rod and communicating with the air channel of the head, an air conduit formed in the cavity, wherein the cavity communicates with the air channel of the head, characterized in that the bending section includes at least one collapsible guide line formed thereon to match with an output power of the air compressor, wherein the at least one collapsible guide line is a boundary axis of the acting area and the positioning zone of the air stop sheet so that a top surface of the air stop sheet facing the cylinder forms an obtuse angle less than 180 degrees, and a back surface of the acting zone of the air stop sheet backing a top of the cylinder turns on relative to a plane of a top of the head at an open angle θ, thus producing an air flowing space; the cavity has a column extending from a bottom thereof, and a spring fitted on the column and inserting through the air channel of the head along the cavity of the piston rod so that the spring abuts against a back surface of an acting zone of an air stop sheet, wherein the air stop sheet is forced by the spring to locate in the acting zone at the open angle θ, and the air conduit and the air channel communicate with atmosphere. wherein the piston of the air compressor includes an air stop sheet mounted on a head thereof, and the air stop sheet has a bending section, wherein the bending section has a positioning zone arranged on the head, and the bending section further has an acting zone located opposite to the positioning zone and configured to cover an air channel of the piston; wherein a piston rod extends downward from the head, and the piston rod includes a cavity horizontally passing through a top of the piston rod and communicating with the air channel of the head, an air conduit formed in the cavity, wherein the cavity communicates with the air channel of the head, characterized in that the bending section includes at least one collapsible guide line formed thereon to match with an output power of the air compressor, wherein the at least one collapsible guide line is a boundary axis of the acting area and the positioning zone of the air stop sheet so that a top surface of the air stop sheet facing the cylinder forms an obtuse angle less than 180 degrees, and a back surface of the acting zone of the air stop sheet backing a top of the cylinder turns on relative to a plane of a top of the head at an open angle θ, thus producing an air flowing space; the cavity has a column extending from a bottom thereof, and a spring fitted on the column and inserting through the air channel of the head along the cavity of the piston rod so that the spring abuts against a back surface of an acting zone of an air stop sheet, wherein the air stop sheet is forced by the spring to locate in the acting zone at the open angle θ, and the air conduit and the air channel communicate with atmosphere.