A lamp, having a lampshade, a lamp holder, a connective structure, a light source component, and a power supply component; the lampshade has a cover body and a mounting seat; one end being defined as a first end of the connective structure is connected with the lampshade, with another end being defined as a third end connected with the lamp holder; the mounting seat is mounted to the first end; the lamp holder is fitted with the third end; a rotation prevention structure between the mounting seat and the first end is given to prevent rotation of the lampshade relative to the connective structure; the rotation prevention structure contains anti-rotation ribs and anti-rotation grooves corresponding to the anti-rotation ribs; the anti-rotation ribs are fitted in the anti-rotation grooves.