A chair having a fold-down tablet assembly, comprising: a board; a torsion positioning mechanism, fixed to a side of the board for horizontally rotating and positioning the board; a shaft, having a protrudent positioning bar, and fixed to the torsion positioning mechanism for making the board vertically rotatable; and an armrest, having a shaft hole for rotatably receiving the shaft and a slot communicating with the shaft hole, wherein when the board is turned horizontally, the positioning bar is embedded into the slot to be restricted; characterised in that the torsion positioning mechanism comprises: a casing, fixed on the board, having a cavity and a round hole, wherein an edge of the cavity is formed with a stopping recess, and two opposite sides of the cavity are separately formed with a first blocking portion and a second blocking portion, which are located aside the round hole; a seat, connected to the shaft, formed with a pivot base corresponding to the round hole, wherein the pivot base is axially inserted into the round hole so that the casing can drive the board to rotate on the pivot base; a restriction plate, correspondingly disposed over the round hole, and having a protrudent positioning plate with an inserting hole formed on an edge of the restriction plate; and a torsion spring, received in the cavity, and having a coil, a first arm fixed in the stopping recess and a second arm fixed in the inserting hole; wherein when the board is in a horizontally positioning status, the torsion spring is pressed to be immovable, when the board is being folded, push the board outwards to make the torsion spring extend for assisting the board to automatically rotate outwards horizontally, and the board is rotated only within an angular range defined between the two blocking portions (1214, 1215). a board; a torsion positioning mechanism, fixed to a side of the board for horizontally rotating and positioning the board; a shaft, having a protrudent positioning bar, and fixed to the torsion positioning mechanism for making the board vertically rotatable; and an armrest, having a shaft hole for rotatably receiving the shaft and a slot communicating with the shaft hole, wherein when the board is turned horizontally, the positioning bar is embedded into the slot to be restricted; characterised in that the torsion positioning mechanism comprises: a casing, fixed on the board, having a cavity and a round hole, wherein an edge of the cavity is formed with a stopping recess, and two opposite sides of the cavity are separately formed with a first blocking portion and a second blocking portion, which are located aside the round hole; a seat, connected to the shaft, formed with a pivot base corresponding to the round hole, wherein the pivot base is axially inserted into the round hole so that the casing can drive the board to rotate on the pivot base; a restriction plate, correspondingly disposed over the round hole, and having a protrudent positioning plate with an inserting hole formed on an edge of the restriction plate; and a torsion spring, received in the cavity, and having a coil, a first arm fixed in the stopping recess and a second arm fixed in the inserting hole; wherein when the board is in a horizontally positioning status, the torsion spring is pressed to be immovable, when the board is being folded, push the board outwards to make the torsion spring extend for assisting the board to automatically rotate outwards horizontally, and the board is rotated only within an angular range defined between the two blocking portions (1214, 1215). a casing, fixed on the board, having a cavity and a round hole, wherein an edge of the cavity is formed with a stopping recess, and two opposite sides of the cavity are separately formed with a first blocking portion and a second blocking portion, which are located aside the round hole; a seat, connected to the shaft, formed with a pivot base corresponding to the round hole, wherein the pivot base is axially inserted into the round hole so that the casing can drive the board to rotate on the pivot base; a restriction plate, correspondingly disposed over the round hole, and having a protrudent positioning plate with an inserting hole formed on an edge of the restriction plate; and a torsion spring, received in the cavity, and having a coil, a first arm fixed in the stopping recess and a second arm fixed in the inserting hole; wherein when the board is in a horizontally positioning status, the torsion spring is pressed to be immovable, when the board is being folded, push the board outwards to make the torsion spring extend for assisting the board to automatically rotate outwards horizontally, and the board is rotated only within an angular range defined between the two blocking portions (1214, 1215).