A main shaft locking mechanism includes a fixing ring, a main shaft and detents. The main shaft includes an equilateral portion that has an equilateral cross section. A number of the corners of the equilateral portion is equal to that of the detents. The detent has an exterior side and an interior side opposite to the exterior side. The interior side includes two corner protrusions protruded out from two ends thereof and a concave portion defined between the two corner protrusions. The corners of the main shaft are received in the concave portions of the detents. When the main shaft is manually driven to rotate, the corners of the main shaft drive the detents to deviate. Therefore, the exterior sides of the detents interfere against the fixing ring. Then the detents prevent the main shaft from further rotating, thus locking up the main shaft.