Gentle zones were found in all output power curves of laser-diode (LD) side-pumped solid-state lasers. Through the calculation of the radial and tangential thermal lens effect, the gentle zones were found at the boundary of the radial stability zones, going through the gentle zone, the output power increased continuously and reached its highest value at the boundary of the tangential stability zone with the lowest M2 value. The gentle zones were thought to be due to the combined effects of the radial and tangential stability zones and high-brightness lasers can be obtained at the stable power point. Based on this, a simple high-brightness laser with a cavity length of 200 mm was designed to operate at the boundary of the tangential stability zone, and laser at 1064 nm with output power of 50 W and theoretical beam factor M2=2 was achieved when the pump power was 220 W.