The invention discloses a telescopic beam, an adjustable mold base system made by using the same and a mold base construction method. The telescopic beam comprises two n-shaped steel beams and an adjusting beam arranged between the two n-shaped steel beams to play a connection role; each n-shaped steel beam has an upward notch, and flanges at two sides of the notch are correspondingly provided with a plurality of groups of prepared bolt holes; the adjusting beam has a U-shaped cross section; the bottom grooves at the end parts of the n-shaped steel beams at two sides are partially embedded inside the U-shaped groove of the adjusting beam; two side walls of the adjusting beam are correspondingly provided with a plurality of groups of adjusting holes; and after the length of the telescopic beam is adjusted to place, the adjusting holes and the bolts in the prepared bolt holes are used for realizing adjustable connection between the adjusting beam and the n-shaped steel beams. The telescopic beam of the invention can be widely applied to a building construction mold base supporting system, material is saved, durability is achieved, and maintenance and disassembly are convenient.