A light blocking system (480, 580) adapted to be used in a vertical blind (400, 500, 600, 700) which comprises a headrail (410, 510, 610, 710), a control mechanism (430, 530, 630, 730), and a covering assembly, wherein a gap (450, 550, 650, 750) is formed between the headrail (410, 510, 610, 710) and the covering assembly; the control mechanism (430, 530, 630, 730) comprises an operable portion, a motion portion, and a plurality of clips, wherein the motion portion is provided in the headrail (410, 510, 610, 710); the covering assembly comprises a plurality of slats (420, 520, 620, 720), wherein each of the slats (420, 520, 620, 720) is suspended from one of the clips with an end thereof, and therefore is hung below the headrail (410, 510, 610, 710); each of the clips is rotatably connected to the motion portion; the operable portion is exposed out of the headrail (410, 510, 610, 710) to be operated, and drives the motion portion when operated, whereby to turn the slats (420, 520, 620, 720) in situ relative to the headrail (410, 510, 610, 710), 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 (420, 520, 620, 720) overlaps each other to block out light, wherein a surface of each of the slats (420, 520, 620, 720) 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 (420, 520, 620, 720) is substantially perpendicular to a longitudinal direction of the headrail (410, 510, 610, 710), which allows a most amount of light to pass through; the slats (420, 520, 620, 720) are operable to move back and forth in the longitudinal direction of the headrail (410, 510, 610, 710); the light blocking system (480, 580) comprises a cover plate (482, 484, 582, 584, 682, 684, 782) and a driving module (486, 586, 686, 786), wherein the cover plate (482, 484, 582, 584, 682, 684, 782) is disposed corresponding to the gap (450, 550, 650, 750), and covers the gap (450, 550, 650, 750) when the covering assembly is in the closed state; the cover plate (482, 484, 582, 584, 682, 684, 782) is movable along with operation of the control mechanism (430, 530, 630, 730); when the slats (420, 520, 620, 720) are driven to turn by the control mechanism (430, 530, 630, 730), the cover plate (482, 484, 582, 584, 682, 684, 782) is also driven by the control mechanism (430, 530, 630, 730) to cover or expose the gap (450, 550, 650, 750); the driving module (486, 586, 686, 786) is connected to the control mechanism (430, 530, 630, 730), wherein, when the control mechanism (430, 530, 630, 730) drives the slats (420, 520, 620, 720) to turn, the driving module (486, 586, 686, 786) is also driven by the control mechanism (430, 530, 630, 730) to move the cover plate (482, 484, 582, 584, 682, 684, 782); the light blocking system (480, 580) is characterized in that: the motion portion comprises a led wheel (432, 632), a link wheel, and a link shaft (438, 638, 736); each of the clips is rotatably connected to the link shaft (438, 638, 736); the operable portion is adapted to be operated to drive the led wheel (432, 632) to rotate, and the led wheel (432, 632) drives the link wheel to rotate while being driven by the operable portion, so as to drive the link shaft (438, 638, 736) to rotate as well, whereby to turn the slats (420, 520, 620, 720) in situ relative to the headrail (410, 510, 610, 710); the driving module (486, 586, 686, 786) comprises a mounting portion (4862, 5862, 6864, 7866) and a pushing portion (4864, 4866, 5864, 6866, 7864); the mounting portion (4862, 5862, 6864, 7866) is provided corresponding to the link shaft (438, 638, 736), and is synchronously movable with the link shaft (438, 638, 736); the pushing portion (4864, 4866, 5864, 6866, 7864) extends outward from the mounting portion (4862, 5862, 6864, 7866), and a free end of the pushing portion (4864, 4866, 5864, 6866, 7864) optionally abuts against the cover plate (482, 484, 582, 584, 682, 684, 782); a maximum distance between the mounting portion (4862, 5862, 6864, 7866) and the free end of the pushing portion (4864, 4866, 5864, 6866, 7864) is greater than a distance between the mounting portion (4862, 5862, 6864, 7866) and the cover plate (482, 484, 582, 584, 682, 684, 782) when the cover plate (482, 484, 582, 584, 682, 684, 782) completely covers the gap (450, 550, 650, 750); when the mounting portion (4862, 5862, 6864, 7866) rotates, the mounting portion (4862, 5862, 6864, 7866) drives the pushing portion (4864, 4866, 5864, 6866, 7864) to optionally push against the cover plate (482, 484, 582, 584, 682, 684, 782), so that the cover plate (482, 484, 582, 584, 682, 684, 782) is pivotable relative to the headrail (410, 510, 610, 710) to cover or expose the gap (450, 550, 650, 750).