Abstract: The high-pressure jet method, MJS, was applied in the reinforcement of the timber pile foundation located at Ming-Qing ancient seawall based on the research subject on protecting ancient seawall of Qiantang River from flood. In order to investigate the performance properties of timber pile foundation reinforced by MJS method, the monitoring on the deformation of seawall and tests on the treatment effect were carried out on the field. The deformation monitoring includes the settlement and horizontal displacement of seawall, while the tests on the treatment effect consist of the cone penetration test and single-pile static load test. The deformation monitoring shows that the settlement of seawall reinforced by the MJS is small, however, the horizontal displacement is obvious. The tests indicate that the diversity of the radius of pile constructed by MJS method is more than 1.5m; the mechanical strength of soil is obviously improved; the slope of the load-settlement curve of the reinforced timber pile is slowed down, while the bearing capacity of singe timber pile is increased by more than 60%.