The water level in the coastal areas will fluctuate with the tides,and by doing field monitoring of the soil in the region of the soft base and the changing areas of the water level,we study the effects of water level changes on the surface of the earth,the pressure of the pore water,and the effects of the process.The result shows that the water pressure dissipation is incomplete in the soil in the changing area of the water level,the receding tide causes the drop of the water level,the soil will form a dark surge in the discharge of water,and the small particles in the soil will be taken out to weaken the stability and strength the soil.So,we shall consider the change of the field’s water level and may take measures such as filling a cofferdam and building a clay sealing wall along the field line,driving the steel sheet pile,or arranging evenly the dewatering well on the edge or in the middle of the field to avoid the influence of the change of underground water level on the engineering quality.