A mechanical seal for liquid with an axially floating tray sleeve inside, comprising a movable ring, a static ring, a locking ring and a tray sleeve, wherein the static ring is connected to a cover and a shell to be stationary, the movable ring is fixed with a shaft or a shaft sleeve through the locking ring and rotates, and the contact end faces of the movable ring and the static ring form a sealing face which is vertical to and slides against the shaft; the tray sleeve is between a sealing ring and the shaft or shaft sleeve in the direction vertical to the shaft; the tray sleeve is connected to the movable ring or the static ring near an inner end of the liquid in the axial direction; the tray sleeve extends out of the cover in the axial direction near the air end.