A scheme to efficiently generate a broadband short attosecond pulse is presented by using intense multi-cycle 800 nm laser in combination with its 27th harmonic (H27) laser pulse. It is shown that the electronic dynamics can be efficiently controlled by the 1 fs H27 laser. Setting the H27 laser to the multi-cycle 800 nm laser at a proper time, we can control most of the ionizations to occur at half the optical cycle, and the short quantum path can be picked out, thus the harmonic plateau is heightened four orders of magnitude compared with the case of using the fundamental laser alone. Furthermore, a harmonic supercontinuum spectrum with a bandwidth of 108 eV is produced. By superposing the 140th-210th harmonics in the continuous region, an intense isolated pulse with the duration of 39 as is straightforwardly obtained. Using this method, the bandwidth of the continuum spectrum is 2 times wider, the intensity of the attosecond pulse is greater, and the duration of the attosecond pulse is shorter, in contrast to the scheme proposed in Ref. [7], which added the ultraviolet attosecond pulse to a few-cycle 800 nm laser.