The invention relates to a cementitious consolidating anti-seepage agent for clay of an anti-seepage system at the bottom of a landfill. The cementitious consolidating anti-seepage agent is prepared from, by mass, 78-82% of modified fly ash, 14-16% of a cementitious material and 4-6% of polyvinyl alcohol, and a method for preparing the modified fly ash includes the steps of: drying blast furnace slag, grinding the dried blast furnace slag into fine particles, sifting the ground slag by using a sieve with 2 mm meshes so as to obtain 0.5 part of sifted slag, mixing 1 part of fly ash with 0.5 part of the sifted slag, performing even stirring to obtain a coal slag mixture, calcining the coal slag mixture at a high temperature of 800-830 DEG C for 2 h, and after the calcination, mixing 0.01 part of sodium bicarbonate with the coal slag mixture evenly to obtain the modified fly ash. Through the cementitious consolidating anti-seepage agent for the clay, the tensile performance and durabilityof a clay consolidated layer are enhanced, and the anti-seepage performance and service life are improved.