In order to obtain high-intensity water window spectral continuum, a method to control the laser waveform by using frequency-chirp field is proposed. It is found that the harmonic cutoff can be extended in the presence of single-color multi-cycle negative chirped pulse. With the introduction of the second negative chirped pulse, not only the harmonic cutoff can be extended to the water window region, but also the harmonic yield can be enhanced by nearly three orders of magnitudes. When the third 123 nm ultraviolet pulse is added, the harmonic yield can be further improved due to ultraviolet resonance ionization, and the enhancement of harmonic yield is nearly independent of the chirp and laser phase of the third pulse. Finally, with the superposition of the harmonics on the water window spectral continuum, a 38 as pulse can be obtained.