We theoretically study the high-order harmonic generation (HHG) of a model He atom exposed to a combined field of 1600 nm and 800 nm laser pulses. The changing of rising rate and falling rate with time for the combined filed during main HHG, can present the different characteristics of high-order harmonic generation from the short electron trajectories and the long electron trajectories. By superimposing several harmonics generated from He atom driven by the combined field, we could obtain shorter attosecond pulses. Finally, by adjusting the time delay between two laser pulses, we can effectively suppress the contribution of the short electron trajectories to HHG, and obtain an isolated attosecond pulse with duration 33.7 as.