The invention provides a yarn guide plate adjusting device of a hosiery knitter. The yarn guide plate adjusting device of the hosiery knitter comprises a sinker cap, a rotating ring, multiple yarn guide plates, multiple elastic pieces and a driving piece, wherein the rotating ring is sleeved on the sinker cap; the yarn guide plates are sleeved on the sinker cap; the elastic pieces are elastically propped between the yarn guide plates and the sinker cap; and the driving piece is pivoted on the sinker cap. The driving piece is pushed by using outside force so that the rotating ring can be driven to rotate; the yarn guide plates are clamped by using multiple clamping parts of the rotating ring so that the yarn guide plates synchronously move in the radial direction of the sinker cap to realize the purpose of adjusting the positions of the yarn guide plates; the yarn guide plates can be prevented from cracking a knitting needle to affect the quality of the hosiery due to displacement during knitting.