The invention discloses an automatic chain tensioning structure comprising outer link plates, inner link plates, pins, sleeves, rolling posts, grooves, tensioning springs or torsion springs and the like. In other words, one groove used for a pin moving longitudinally is formed in each inner link plate of various existing chains, and one tensioning spring mounted at the rear end of each groove is used for always tensioning the pin automatically. The automatic chain tensioning structure has the advantages that each chain and sprocket are always tensioned automatically, the number for performing tensioning adjustment of the chain cam be reduced effectively, and service life of the chain, the sprocket or other parts can be prolonged greatly; the automatic chain tensioning structure is simple in structure, low in cost and absolutely safe, reliable and practical, is adaptable to optimization, mounting and using various chains, and is worth vigorous promotion.