A casing nipple and a wellhead pressure relief device, the casing nipple comprising: a nipple body having an inner passage on which a plurality of liquid inlet passages are provided at intervals in acircumferential direction thereof; the centralizing convex parts are arranged on the outer wall of the pup joint body at intervals, the upper parts of the centralizing convex parts are opposite to theliquid inlet channel, liquid inlet holes are formed in the lower ends of the centralizing convex parts, the centralizing convex parts are provided with central channels, and the liquid inlet holes are communicated with the liquid inlet channel through the central channels; each one-way pressure reducing mechanism is arranged in the central channel of each centralizing convex part; the one-way pressure reducing mechanism is provided with a one-way pressure reducing valve which is sequentially provided with an elastic piece, a pressing cap and a valve seat from top to bottom, one end of the pressing cap sleeves the elastic piece, and the other end of the pressing cap can be in sealing fit with the valve seat; and the valve bonnet is connected into the central channel and abuts against the valve seat. Annular pressure between sleeves can be released, high-pressure fluid in the outer annulus enters the inner annulus through the one-way pressure reducing mechanism, and meanwhile the fluidis prevented from flowing back from the inner annulus to the outer annulus.