The invention discloses a device and method for the online leakage detection of a buried water conveying pipeline. The device comprises a leaking water collecting device, a salt bag and two detection lines, wherein the leaking water collecting device is arranged outside the water conveying pipeline, the salt bag is arranged in the leaking water collecting device, and the two detection lines are used for detecting the conductivity of leaking water; the salt bag is arranged at the bottom of the leaking water collecting device and contains a corrosion-resistant soluble electrolyte; first ends of the detection lines are fixedly arranged at the bottom of the leaking water collecting device, and second ends of the detection lines are led out to a ground detection point. The method comprises the steps of measuring the conductivity between the two detection lines of the detection point by using a conductivity meter, and calculating the volume of the leaking water according to a salt content-conductivity relationship at given water temperature and the salt content of the salt bag. According to the device and the method, through arranging the leaking water collecting device, the salt bag and the detection lines, leakage points and leaking water volume can be detected accurately on the premise of not causing any influence on normal water conveying by using the quantitative relationship between salt content and conductivity of the salt soluble electrolyte in an aqueous solution state.