A light blocking system adapted to be used in a vertical blind which comprises a headrail , a control mechanism , and a covering assembly, wherein a gap is formed between the headrail and the covering assembly; the control mechanism comprises an operable portion, a motion portion, and a plurality of clips, wherein the motion portion is provided in the headrail ; the covering assembly comprises a plurality of slats , wherein each of the slats is suspended from one of the clips with an end thereof, and therefore is hung below the headrail ; each of the clips is rotatably connected to the motion portion; the operable portion is exposed out of the headrail to be operated, and drives the motion portion when operated, whereby to turn the slats in situ relative to the headrail , so that the covering assembly is adapted to be sequentially changed between a closed state, a fully open state, and another closed state; when the covering assembly is in the closed state and the another closed state, the slats overlaps each other to block out light, wherein a surface of each of the slats faces opposite directions when the covering assembly is in the closed state and in the another closed state; when the covering assembly is in the fully open state, each of the slats is substantially perpendicular to a longitudinal direction of the headrail , which allows a most amount of light to pass through; the slats are operable to move back and forth in the longitudinal direction of the headrail ; the light blocking system comprises a cover plate and a driving module , wherein the cover plate is disposed corresponding to the gap , and covers the gap when the covering assembly is in the closed state; the cover plate is movable along with operation of the control mechanism ; when the slats are driven to turn by the control mechanism , the cover plate is also driven by the control mechanism to cover or expose the gap ; the driving module is connected to the control mechanism , wherein, when the control mechanism drives the slats to turn, the driving module is also driven by the control mechanism to move the cover plate ; the light blocking system is characterized in that : the motion portion comprises a led wheel , a link wheel , and a link shaft ; each of the clips is rotatably connected to the link shaft ; the operable portion is adapted to be operated to drive the led wheel to rotate, and the led wheel drives the link wheel to rotate while being driven by the operable portion, so as to drive the link shaft to rotate as well, whereby to turn the slats in situ relative to the headrail ; the driving module comprises a mounting portion and a pushing portion ; the mounting portion is provided corresponding to the link shaft , and is synchronously movable with the link shaft ; the pushing portion extends outward from the mounting portion , and a free end of the pushing portion optionally abuts against the cover plate ; a maximum distance between the mounting portion and the free end of the pushing portion is greater than a distance between the mounting portion and the cover plate when the cover plate completely covers the gap ; when the mounting portion rotates, the mounting portion drives the pushing portion to optionally push against the cover plate , so that the cover plate is pivotable relative to the headrail to cover or expose the gap . <EOS>