The high order harmonic generation (HHG) from asymmetric molecular gas in two-color field has been investigated and a method for broadband supercontinuum generation in the plateau by asymmetric molecular gas exposed to multicycle laser pulse is proposed. The numerical simulation shows that the harmonics at the end of the plateau become continuous and form a supercontinuum with a bandwidth of about 80 eV, from which a 50 as pulse in the transform limited case can be generated, when asymmetric molecules are exposed to a multicycle two-color field, which is synthesized by a 10 fs/800 nm fundamental pulse and a 15 fs/1200 nm control pulse. After propagation through the molecular gas, most of the modulations in the supercontinuum are eliminated, and the supercontinuum becomes smoother, from which a 90 as isolated pulse can be produced.