A window shade comprising a shade, a sliding base, a weight track, a first blocking unit and two second blocking units; wherein the shade has two mount bars respectively formed at an upper end and a lower end thereof; wherein the sliding base comprises a first locating groove at a lateral side thereof, and a shade groove being formed at a bottom portion of the sliding base and located under the first locating groove, and the mount bar on the upper end of the shade being adapted to be coupled in the shade groove; wherein the weight track is an elongated board having U-shape cross-section, and a connecting space is formed therein, and wherein the mount bar on the lower end of the shade is configured to be coupled in the connecting space; characterized in that each of two ends of the connecting space has a locating hole formed at a bottom portion thereof; wherein the first blocking unit comprises a connecting block and a first block, an end of the connecting block being coupled in the first locating groove while the other end thereof comprises a first engaging portion; wherein a connecting channel penetrates the first block from one end to the other end, and an outer end of the connecting channel, which is located far from the sliding base than an inner end thereof, comprises a first groove, and a stepped second engaging portion is formed between the connecting channel and the first groove; the connecting block being coupled with the first block, and wherein the first engaging portion is slidably coupled in the connecting channel, and the first block is adapted to have vertical movement relative to the first engaging portion through the connecting channel, so as to enable the first block to block or unblock the shade groove at an end thereof; and wherein each of the second blocking units comprises a cover and a cover board, and one side of the cover is concaved to form a covering space, and the two covering spaces are respectively disposed and coupled on two ends of weight track; each of the covers comprising a through slot at the side opposed to the covering space, wherein the through slot is communicated with the covering space; the cover board comprising an engaging rib and a locating protrude which respectively protrude from a lateral side of the cover board, wherein an end of the engaging rib has an engaging bolt protruding from a bottom portion thereof; the engaging rib and the locating protrude being respectively coupled in and movable along the through slot, and the engaging rib being further inserted into the covering space; and the engaging bolt being engaged with the locating hole of the weight track, so as to secure the cover on the weight track. wherein the shade has two mount bars respectively formed at an upper end and a lower end thereof; wherein the sliding base comprises a first locating groove at a lateral side thereof, and a shade groove being formed at a bottom portion of the sliding base and located under the first locating groove, and the mount bar on the upper end of the shade being adapted to be coupled in the shade groove; wherein the weight track is an elongated board having U-shape cross-section, and a connecting space is formed therein, and wherein the mount bar on the lower end of the shade is configured to be coupled in the connecting space; characterized in that each of two ends of the connecting space has a locating hole formed at a bottom portion thereof; wherein the first blocking unit comprises a connecting block and a first block, an end of the connecting block being coupled in the first locating groove while the other end thereof comprises a first engaging portion; wherein a connecting channel penetrates the first block from one end to the other end, and an outer end of the connecting channel, which is located far from the sliding base than an inner end thereof, comprises a first groove, and a stepped second engaging portion is formed between the connecting channel and the first groove; the connecting block being coupled with the first block, and wherein the first engaging portion is slidably coupled in the connecting channel, and the first block is adapted to have vertical movement relative to the first engaging portion through the connecting channel, so as to enable the first block to block or unblock the shade groove at an end thereof; and wherein each of the second blocking units comprises a cover and a cover board, and one side of the cover is concaved to form a covering space, and the two covering spaces are respectively disposed and coupled on two ends of weight track; each of the covers comprising a through slot at the side opposed to the covering space, wherein the through slot is communicated with the covering space; the cover board comprising an engaging rib and a locating protrude which respectively protrude from a lateral side of the cover board, wherein an end of the engaging rib has an engaging bolt protruding from a bottom portion thereof; the engaging rib and the locating protrude being respectively coupled in and movable along the through slot, and the engaging rib being further inserted into the covering space; and the engaging bolt being engaged with the locating hole of the weight track, so as to secure the cover on the weight track.