The time-frequency properties of the high-order harmonic generation in the presence of a static electric field are investigated. It is found that an attosecond pulse train with one burst per optical cycle can be extracted by superposing the harmonics near the cutoff. This characteristic can be used to generate isolated attosecond pulse in multi-cycle regime. When the ratio of the laser field to the static field is 0.39, the harmonic spectrum is extended to Ip＋9.1Up, and the harmonics above Ip＋0.7Up are emitted almost in phase. The phase-locked harmonics covering a broad bandwidth are produced, and then a regular attosecond pulse train with the pulse duration less than 100 as is generated.