A light-emitting device and a related projection system. Light-emitting devices (300, 400) comprise: a light source unit (1) and focusing lenses (330, 430). The light source unit (1) comprises laser units (310, 410) and collimating lenses (320, 420) corresponding to the laser units (310, 410). The light-emitting surface of the laser units (310, 410) is a rectangle, and the light divergence angle of laser passing through the cross-section of the long side of the rectangle is smaller than the light divergence angle of same passing through the cross-section of the short side of the rectangle. The collimating lenses (320, 420) are used for focusing the laser from the laser units (310, 410) on a target surface to form a predetermined light spot. The laser units (310, 410) are located on the light axis of the collimating lenses (320, 420) in a predetermined position deviating from the focal point thereof, so that the predetermined light spot has a predetermined length-width ratio. The light-emitting device and the related projection system have the advantages of relatively small regulation quantity and relatively high assembly efficiency.