We propose an efficient method to generate an ultrashort attosecond pulse when a model He is exposed to the combination of an intense few-cycle chirped laser pulse and a half cycle pulse. By solving the time-dependent Schrdinger equation numerically, we find that the cut-off energy of the harmonics is extended effectively to Ip+21.6Up. By superimposing some high-order harmonics in different regions for the second plateau, the obtained pulses are all single attosecond ones. Minimum pulse achieves 37 as. Especially, by superimposing the lower order harmonics of the second plateau, one can obtain single attosecond pulse, and also the intensity of the single pulse is three order of magnitude higher than the attosecond pulse obtained near cut-off of harmonics.