Air compressor

An air compressor, comprising:

• a cylinder having a piston passage, wherein the piston passage has a rear end and a front end opposite to each other;
• a piston comprising a piston portion and a shaft portion, wherein the piston portion is located in the piston passage and has an annular groove, and the shaft portion is connected to the piston portion via the rear end;
• a piston ring disposed at the annular groove, wherein a first portion of the piston ring is located at a first side portion of the piston portion, and a second portion of the piston ring is located at a second side portion of the piston portion;
• and
• a driving unit coupled to the shaft portion and adapted to drive the piston portion to move back and forth between the front end and the rear end via the shaft portion,
• wherein a width of the annular groove at the first side portion is greater than a width of the annular groove at the second side portion, such that the first portion of the piston ring is movable along a width direction of the annular groove as the piston portion moves back and forth.