Quasi-CW laser (QCW) can operate in both continuous wave and high peak power pulse modes. Different from the traditional CW laser, its peak and average power are always the same in CW and CW/modulation modes, while the peak power of QCW laser in pulse mode is several times or even 10 times higher than the average power. QCW can generate high-energy microsecond and millisecond pulses at repetition frequency from tens hertz to thousands hertz, achieving thousands watts of average power and peak power. Therefore, it is increasingly used in precision welding.