This invention relates to a plant irrigation system and in particular to one including a reservoir for receiving water, having an upper indicating line and a lower indicating line on the outer surface thereof for denoting the highest and lowest water levels respectively, an inlet on the top thereof for filling water therein, a cap engageable with the inlet for preventing water from evaporating and flowing out thereof, an outlet on the top thereof, a first mounting plate arranged on the top thereof and having two openings, and a second mounting plate formed on the top thereof and having two slits; a hook having two protuberances engageable with the two openings of the first mounting plate so that the hook is detachably connected with the reservoir and can be adapted for use to various flower pots; a plastic pipe having a lower end to be extended into the reservoir through the outlet of the reservoir and an upper end to be buried into the soil in a flower pot; a wick enclosed within the plastic pipe; and a spade for digging the soil, with a flexible and wrinkly strap for being firmly inserted through the slit of the second mounting plate and a fastening means designed for keeping the plastic pipe in position, burying the plastic pipe into the soil, and providing further fixation for the reservoir onto the flower pot; whereby the plant irrigation system utilizes capillary action to transmit water from the reservoir to a plant in a flower pot via the wick enclosed in the plastic pipe thereby automatically controlling and adjusting required amount of water to meet different demands of various plants in different living environments.