A scheme to efficiently generate an intense short isolated attosecond pulse is presented by the combined laser pulse irradiating the coherent superposition state. The combined pulse is composed of a multi-cycle 800 nm laser and a 1600 nm one. Compared with the case of the fundamental laser irradiating the ground state, the cutoff position is significantly extended from 70th to 280th harmonic, a broadband supercontinuum spectrum with 108 eV spectral width can be produced which directly generates an isolated 38 attosecond pulse, the intensity of which is increased eleven orders of magnitude. Our simulation also shows that the value of the time delay has a large range to keep the isolated sub-50-as pulse generation. Moreover, the value of the time delay in the range between ±0.05π is optinal. This result shows that the scheme appears feasible for an experimental demonstration.