We propose a new method to directly generate an isolated sub-50 attosecond pulse. The combination of a 5fs driving laser pulse with the wavelength of 800nm and a low frequency field with the wavelength of 10μm is adopted. The intensity of the low frequency field is 10% that of the driving pulse. It is shown that the harmonic spectrum is extended to Ip+8.2Up and the harmonics above Ip+4.2Up become continuous. By adjusting the phase of the carrier-envelope of the few-cycle pulse，the bandwidth of the supercontinuum can be further broadened and a close-to-Fourier-limit sub-50as pulse is directly obtained.