The present invention provides an eccentric roundel structure for three-compressing-chamber diaphragm pump. The eccentric roundel structure is a truncated-cylinder eccentric roundel in an eccentric roundel mount. The truncated-cylinder eccentric roundel characteristically comprises an annular positioning dent, a truncated cylinder peripheral and a sloped top ring created from the annular positioning dent to the truncated cylinder peripheral to replace a conventional rounded shoulder. By means of the sloped top ring, the oblique pull and squeezing phenomena of high frequency incurred by the rounded shoulder in a conventional tubular eccentric roundel are completely eliminated. Thus, not only the durability of the three-compressing-chamber diaphragm pump for sustaining the pumping action of high frequency from the truncated-cylinder eccentric roundels is mainly enhanced but also the service lifespan of the three-compressing-chamber diaphragm pump is exceedingly prolonged.