Leakage detection for water distribution systems based on acoustic techniques are prevalent in recent years. However, few work has been done in the acoustic characteristic analysis of metal pipes using in-pipe sensors. Distinguishing itself by the feather of continuous measurement in spatial scale, the in-pipe detection has an advantage of high sensitivity. This paper is concerned with experimental investigation on acoustic characteristics of small leakage in metal water pipe. Leakage signals at different distances from leakage source were measured to reflect the continuity of space. Results show that the leakage signals drop to the level of background noise above 3 kHz in frequency domain. Four characteristic bands are extracted to analyze the acoustic feature of leakage signals in the studied frequency band. It can also be concluded that the effective in-pipe detection distance is at least 1.5 m, and within this distance, the leakage signal barely decays. Within the water pressure range of 0.1-0.2 MPa, the energies of the characteristic bands descend on an average speed of 103 dB/MPa as pressure drops. Further field tests will be conducted using an in-pipe detection prototype.