By solving the time-dependent Schrdinger equation with split-operator method, we study the high harmonics of one-dimensional helium atom exposed to the two-color laser field composed of IR field and UV field, and analyze the characteristics of the attosecond pulse produced by the high-order harmonics. It is found that the width of the attosencond pulse is shortened by the increasing intensity of IR field in the combined laser field. By adjusting the time delay between two lasers, we can enhance the contribution of one of the electron trajectories and suppress that of the other. Thus we can obtain an isolated sub-100 as pulse.