The present invention relates to a light-shielding apparatus for a plastic greenhouse, capable of easily unfolding or folding a light-shielding net through a simple structure. The light-shielding apparatus for a plastic greenhouse comprises an upper guide part, a side guide part, and a light-shielding net. The upper guide part includes a plurality of upper guide coupling elements, which are installed at regular intervals before and after on an upper horizontal frame of the plastic green house. Each of the upper guide coupling elements has a tube shape, of which the front, rear, and lower portion are opened. An upper guide line is installed to pass back and forth inside the upper guide coupling element. Line check units are installed on both left and right sides of the opened lower portion of the upper guide coupling element such that the line check units are able to rotate. The downward separation of the upper guide line is prevented by the line check units on the left and right sides and only a plurality of upper loops inserted into the upper guide lines are allowed to pass back and forth. The side guide part includes a plurality of left and right side guide coupling elements, which are installed at regular intervals before and after on left and right horizontal frames of the plastic green house. Each of the left and right horizontal frames has a tube shape, of which the front, rear, and one portions are opened. A side guide line is installed to pass back and forth inside the side guide coupling element. Line check units are installed above and below the opened portion of the side guide coupling element such that the line check units are able to rotate. The separation of the side guide line to one side is prevented by the line check units above and below and only a plurality of side loops inserted into the side guide lines are allowed to pass back and forth. The upper middle portion of the light-shielding net is connected to the upper loops of the upper guide lines
