We theoretically investigate the high-order harmonic and attosecond pulse generation by numerically solving the one-dimensional time-dependent Schrödinger equation from a hydrogen atom in a two-color laser field, which is synthesized by adding a suitable multicycle infrared pulse to a multicycle 800 nm fundamental pulse. Our results clearly show that when there are 12 Ti: sapphire optical cycles in the pulse envelope, by adding a suitable second optical field, the electric field of the synthesized pulse presents three-segments, and only amplitude of the electric field in middle segment makes a major contribution to the plateau and cutoff region of the harmonic spectrum. By analyzing the compression mechanism of two-color electric field, we further enlarge the duration of the synthesized pulse to 60 fs, and obtain a single 160 as short pulse. This has been the longest pulse duration used for obtaining single attosecond pulse so far. Here the effect of synthesized pulses is similar to the effect of single 5 fs ultrashort pulse. This scheme greatly reduces the requirements for the pump laser system used traditionally for generating an isolated attosecond pulse, and it allows us to use a well-established conventional high-power pumping laser.