We have investigated the characteristics of high harmonic and attosecond pulse generated by helium atom exposed to the combined field of two laser pulses with the same color and a midinfrared laser pulse in nano-structure plasmon by solving the one-dimensional time-dependent Schrödinger equation. It is shown that the cut-off position of the harmonics is obviously extended in the combined field, however, the plasmon structure determined effect of absorbing the electron in the continuum states has an important influence on the cut-off position of the harmonics. It is found that the contribution from some single trajectory of electron is enhanced and other trajectories are suppressed by changing the plasmon position, so a single isolated attosecond pulse is obtained. Comparison with the scenario of atoms driven in homogeneous field, the width of the pulse decreases greatly, and an isolated pulse is achieved to be as short as 28 attoseconds.