The invention discloses a purification treatment system for a riverway. The purification treatment system comprises at least one riverway ecological dam (1) for removing nitrogen, phosphorus and organic matters in water, an aeration device (2) arranged behind the riverway ecological dam (1), an oil collecting and concentrating pool (1) for collecting oils and a floating oil guide device (3) for isolating the oils in the aeration device, wherein the floating oil guide device (3) is arranged between the aeration device (2) and the oil collecting and concentrating pool (7); the oils isolated by the floating oil guide device (3) are guided into the oil collecting and concentrating pool (7) through an oil collecting pipe (8). The invention also relates to a method for purifying water quality by using the purification treatment system for the riverway. The method is simple in structure, low in cost and good in operability and can be used for removing nitrogen, phosphorus, organic matters and the like in riverway water and carrying out deoiling treatment on the oily substances contained in the riverway water so as to ensure that the treated riverway water is discharged up to the standard.