The invention discloses an automatic thread adjusting and yarn feeding device for a circular knitting machine. The device structurally comprises a yarn feeding support, an upper thread guiding rod isfixedly connected to the middle of the yarn feeding support, three upper thread guiding holes are formed in the upper thread guiding rod, and five slide grooves are uniformly distributed in the surface of the yarn feeding support; slide blocks are slidably connected into the slide grooves, shuttles are fixedly connected into the slide blocks, a driving air cylinder is fixedly connected to the topsof the shuttles, shuttle holes run through the bottoms of the shuttles, a lower thread guiding rod is arranged at the upper portions of the shuttle holes, the two ends of the lower thread guiding rodare fixedly connected with the yarn feeding support, six lower thread guiding holes are uniformly distributed on the lower thread guiding rod, a first wax cylinder is arranged at the lower portion ofthe upper thread guiding rod, and a second wax cylinder is arranged at the lower portion of the lower thread guiding rod. According to the automatic thread adjusting and yarn feeding device, throughthe arrangement of a thread adjusting module and a yarn feeding module, thread adjusting and yarn feeding are simpler and more convenient; through the arrangement of the driving air cylinder, automated thread adjusting and yarn feeding are achieved, and clothes woven by the wax cylinders are smoother.