The present invention relates to an outdoor lamp with an improved watertight mechanism. Its peculiar watertight structure is formed by a lower-cap and an upper-cap, wherein a slope shaped on an anti-water rim of the upper-cap to facilitate rainwater slipping, out of the slope and gravity, directly to the ground. A convex bottom is further formed on the anti-water rim of the upper-cap causing rainwater, being unable to flow inwards cross the convex, to fall directly to the ground, thereby preventing rainwater from streaming to the lower-cap. Combining the slope and convex thereof, rainwater flowing into a joint surface thereby being reduced significantly. Additionally, the joint surface is formed with a two-step structure to curtail capillarity, thereby minimizing rainwater permeating inside to damage the lamp.