The invention discloses a high-power laser high-brightness spectrum synthesis system.The system comprises N plane mirrors, a convergent lens and a photoelectric position detector; each plane mirror comprises a substrate, and the two surfaces of each substrate are a reflection surface and a transmission surface respectively; a light beam lambda 1 with the wavelength lambda 1 is reflected to the reflection surfaces of the plane mirrors MN in turn all the way and output after being reflected by the plane mirrors MN; a light beam with the wavelength lambda i is reflected to the reflection surfaces of the plane mirrors MN in turn all the way and output after being reflected by the plane mirrors MN; N weak lasers penetrating through the plane mirrors MN are focused on the photoelectric position detector after passing through the convergent lens to form N light spots; the optical axes of N incident lasers are adjusted to make the N light spots on the photoelectric position detector coincide, and then spectrum synthesis is realized.The system is applicable to multiple light beams with high brightness, and output of higher power can be realized.