An air compressor, including a cylinder comprising a piston, the piston comprising a piston head and an air stop sheet, wherein the air stop sheet is accommodated on a plane of a top of the head of the piston, wherein the air stop sheet includes a bending section, wherein the bending section has a positioning zone arranged on a first side thereof and located on the plane of the top of the head, and the bending section further has an acting zone arranged on a second side thereof and configured to close an air channel of the piston, wherein the bending section is a boundary line of the acting zone and the positioning zone of the air stop sheet so that a positive surface of the air stop sheet forms an obtuse angle less than 180 degrees, and a back surface of the acting zone of the air stop sheet backing the top of the cylinder turns on relative to the plane of the top of the head at an open angle θ, thus producing an air flowing space; characterized in that the bending section of the air stop sheet has at least one collapsible guideline, the bending section being a boundary line of the acting zone and the positioning zone, a piston rod extends downward from the head, and the piston rod includes a cavity defined on a top thereof, an air conduit defined in the cavity and communicating with the air channel of the head, a column extending from a bottom of the cavity, and a spring fitted on the column and extending through the air channel of the head along the cavity of the piston rod so that the spring abuts against the back surface of the acting zone of the air stop sheet, the air stop sheet is pushed by the spring to turn on at an open angle θ, and the air conduit and the air channel communicate with atmosphere. a cylinder comprising a piston, the piston comprising a piston head and an air stop sheet, wherein the air stop sheet is accommodated on a plane of a top of the head of the piston, wherein the air stop sheet includes a bending section, wherein the bending section has a positioning zone arranged on a first side thereof and located on the plane of the top of the head, and the bending section further has an acting zone arranged on a second side thereof and configured to close an air channel of the piston, wherein the bending section is a boundary line of the acting zone and the positioning zone of the air stop sheet so that a positive surface of the air stop sheet forms an obtuse angle less than 180 degrees, and a back surface of the acting zone of the air stop sheet backing the top of the cylinder turns on relative to the plane of the top of the head at an open angle θ, thus producing an air flowing space; characterized in that the bending section of the air stop sheet has at least one collapsible guideline, the bending section being a boundary line of the acting zone and the positioning zone, a piston rod extends downward from the head, and the piston rod includes a cavity defined on a top thereof, an air conduit defined in the cavity and communicating with the air channel of the head, a column extending from a bottom of the cavity, and a spring fitted on the column and extending through the air channel of the head along the cavity of the piston rod so that the spring abuts against the back surface of the acting zone of the air stop sheet, the air stop sheet is pushed by the spring to turn on at an open angle θ, and the air conduit and the air channel communicate with atmosphere.