Soil-water characteristic curve (SWCC) plays an important role in defining the hydro-mechanical behavior of unsaturated soils. Biochar has the properties of porous structure, high specific surface area and high adsorption. The hydraulic characteristics of biochar-amended soil may change due to the influence of natural environmental factors when applied as the cover layer of landfills. In order to study the effect of biochar content on the water retention behavior of biochar-clay mixture in full suction range, the suction of samples was controlled by vapor equilibrium technique (suction range 3–368 MPa), filter paper method (suction range 0–40 MPa) and pressure plate method (suction range 0–1.5 MPa), and the water content and saturation degree of samples after suction equilibrium were determined. The soil-water characteristic curve of biochar-clay mixtures was obtained in the full suction range. The results showed that: (1) The soil-water characteristic curve in the full suction range of biochar-clay mixtures was effectively expressed by the three suction testing methods. (2) Biochar can affect the water retention behavior of clay, but within a certain range of suction, the water retention behavior of biochar-clay mixtures was also related to the pore structure and the morphology of water in the pores. (3) As measured by the pressure plate method, the air intake value of samples decreased as the biochar content increased. When the suction value was less than the air intake value, a horizontal section appeared in the curve, and the samples were always in the saturated state. The greater the content of biochar, the better the water retention of the sample. (4) The relationship between the water retention capacity of biochar-amended clay and biochar content was explained by the microscopic structure of the biochar-clay mixture and the distribution form of biochar in clay.