A piston of an air compressor, the air compressor including the piston which is actuated by a motor to move upward and downward in a cylinder, wherein the piston of the air compressor includes a head accommodating an air stop sheet, the air stop sheet includes a first bending section which is a boundary line of an acting zone and a 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 a plane of a top of the head at an open angle θ1, thus producing an air flowing space, wherein the acting zone has a noncircular spacing groove configured to separate an external portion and a circular portion, a neck formed on a connection of the external portion and the circular portion, a second bending section formed on the neck in a mechanical working manner, and the second bending section and the circular portion extend upward, such that an axial line between the circular portion and the external portion is the second bending section at an obtuse angle less than 180 degrees, and the second bending section is defined between a bottom of the circular portion and a top of the external portion at an acute angle θ2, wherein the head includes a piston rod extending downward therefrom, and the piston rod has a cavity horizontally passing through a top of the piston rod and communicating with the air channel of the head, and an air conduit formed in the cavity, characterized in that the head further includes a column extending from a bottom of the cavity, 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 of the air stop sheet backing a top of the cylinder and a plane of a top of the head at an open angle θ1, and the air conduit and the air channel communicate with atmosphere, wherein the first bending section of the air stop sheet has at least one collapsible guide line, wherein a first collapsible guide line and a second collapsible guide line are formed on the first bending section, and the first bending section has a positioning zone arranged on a first end of the first bending section of the air stop sheet, and an acting zone arranged on a second end of the first bending section of the air stop sheet. wherein the piston of the air compressor includes a head accommodating an air stop sheet, the air stop sheet includes a first bending section which is a boundary line of an acting zone and a 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 a plane of a top of the head at an open angle θ1, thus producing an air flowing space, wherein the acting zone has a noncircular spacing groove configured to separate an external portion and a circular portion, a neck formed on a connection of the external portion and the circular portion, a second bending section formed on the neck in a mechanical working manner, and the second bending section and the circular portion extend upward, such that an axial line between the circular portion and the external portion is the second bending section at an obtuse angle less than 180 degrees, and the second bending section is defined between a bottom of the circular portion and a top of the external portion at an acute angle θ2, wherein the head includes a piston rod extending downward therefrom, and the piston rod has a cavity horizontally passing through a top of the piston rod and communicating with the air channel of the head, and an air conduit formed in the cavity, characterized in that the head further includes a column extending from a bottom of the cavity, 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 of the air stop sheet backing a top of the cylinder and a plane of a top of the head at an open angle θ1, and the air conduit and the air channel communicate with atmosphere, wherein the first bending section of the air stop sheet has at least one collapsible guide line, wherein a first collapsible guide line and a second collapsible guide line are formed on the first bending section, and the first bending section has a positioning zone arranged on a first end of the first bending section of the air stop sheet, and an acting zone arranged on a second end of the first bending section of the air stop sheet.