High-order harmonics generation (HHG) and isolated attosecond pulse generation of H2+ in a few-cycle 800 nm chirped laser field are theoretically studied. Calculation results show that the HHG can limit the harmonics emitted by two nuclei at the same time through choosing the proper chirped parameters, which decreases the interference in the space and helps to obtain a smooth and continuous HHG flat supported only by single nucleus. An isolated 98 attosecond pulse is generated by synthesizing a spectrum width of 100 orders in the smooth and continuous region with the chirped parameters β=6 and the full width at half maximum τ0=5 fs. Calculation results also verify that when there are two nuclei contributing to the HHG, it is harmful to generate the isolated attosecond pulse. In the discussion, the classical returning energy maps and time frequency analysis are performed to explain the physical mechanism of HHG.