An air compressor including a main frame for mounting a motor, and a cylinder fitted with a piston body, the motor capable of driving the piston body to conduct reciprocating motion to produce in the cylinder compressed air which can enter an air storage container via an exit hole defined at a top wall of the cylinder,: wherein the top wall of the cylinder defines additional exit hole, all the exit holes being regulated by a control mechanism to be opened or closed, the control mechanism including a resilient sheet having a root and a plurality of branches (72, 73, 74)(82, 83, 84)(92, 93, 94) extending from the root and corresponding to the exit holes, each branch (72, 73, 74) (82, 83, 84) (92, 93, 94) having a neck portion (722,732, 742) (822, 832, 842) (922, 932, 942) extending from the root and terminating in a leaf (721, 731, 741)(821, 831, 841)(921, 931, 941) configured to have sizes sufficient for covering the exit holes, wherein the root of the resilient sheet defines a positioning hole being attached to the top wall at a central point, characterized in that each of the branches (72, 73, 74) (82, 83, 84)(92, 93, 94) of the resilient sheet defines a positioning hole (723, 733, 743) at one end of its neck portion (722,732, 742) (822, 832, 842) (922, 932, 942) close to the root, the positioning holes of the branches (72, 73, 74) (82, 83, 84)(92, 93, 94) being attached to the top wall respectively at separate points (P1, P2, P3) located in the periphery of the central point to which the root of the resilient sheet is attached, so that each of the branches (72, 73, 74)(82, 83, 84)(92, 93, 94) of the resilient sheet can be moved individually by compressed air without affecting movements of the other branches, so that the piston body can conduct reciprocating motion more smoothly and thus the performance of the air compressor can be increased, and the control mechanism further includes a plurality of O-rings (41, 51, 61)(43, 53, 63), the O-rings (41, 51, 61)(43, 53, 63) being placed around the exit holes respectively, the branches (72, 73, 74)(82, 83, 84)(92, 93, 94) of the resilient sheet having sufficient elastic forces to enable themselves to be in tight contact with the O-rings (41, 51, 61)(43, 53, 63) to seal the exit holes, respectively. wherein the top wall of the cylinder defines additional exit hole, all the exit holes being regulated by a control mechanism to be opened or closed, the control mechanism including a resilient sheet having a root and a plurality of branches (72, 73, 74)(82, 83, 84)(92, 93, 94) extending from the root and corresponding to the exit holes, each branch (72, 73, 74) (82, 83, 84) (92, 93, 94) having a neck portion (722,732, 742) (822, 832, 842) (922, 932, 942) extending from the root and terminating in a leaf (721, 731, 741)(821, 831, 841)(921, 931, 941) configured to have sizes sufficient for covering the exit holes, wherein the root of the resilient sheet defines a positioning hole being attached to the top wall at a central point, characterized in that each of the branches (72, 73, 74) (82, 83, 84)(92, 93, 94) of the resilient sheet defines a positioning hole (723, 733, 743) at one end of its neck portion (722,732, 742) (822, 832, 842) (922, 932, 942) close to the root, the positioning holes of the branches (72, 73, 74) (82, 83, 84)(92, 93, 94) being attached to the top wall respectively at separate points (P1, P2, P3) located in the periphery of the central point to which the root of the resilient sheet is attached, so that each of the branches (72, 73, 74)(82, 83, 84)(92, 93, 94) of the resilient sheet can be moved individually by compressed air without affecting movements of the other branches, so that the piston body can conduct reciprocating motion more smoothly and thus the performance of the air compressor can be increased, and the control mechanism further includes a plurality of O-rings (41, 51, 61)(43, 53, 63), the O-rings (41, 51, 61)(43, 53, 63) being placed around the exit holes respectively, the branches (72, 73, 74)(82, 83, 84)(92, 93, 94) of the resilient sheet having sufficient elastic forces to enable themselves to be in tight contact with the O-rings (41, 51, 61)(43, 53, 63) to seal the exit holes, respectively.