A one-piece office chair made of plastic with an adjusting apparatus comprises a middle axle seat, a seat frame, a back frame, and two armrests which are formed in one-piece to complete the entire chair frame. Furthermore, an adjusting apparatus is applied to provide supports and elasticity for the chair frame, enabling the back frame to adjust the angle in accordance with users' movements, featuring the present invention as an ergonomic design of comfortable seating with easy assembly and light weight.