The utility model relates to a waterproof conduit seat packer used for offshore drilling, comprising a flange which is provided with circular troughs axially; the lower end of the flange is extended by an internal extension cylinder and an external extension cylinder; the extension internal cylinder is provided with at least two sealing rings; the internal end surface of the circular trough of the flange and the internal side wall of the external extension cylinder are provided with replaceable sealing tyres; three-step sealing is formed by the sealing of the sealing ring on the internal extension cylinder, the sealing of the internal end surface of the circular trough and the sealing of the external extension cylinder. The waterproof conduit seat packer has the advantages that the upper lifting of a returning pipe is centered by the clamping effect of the internal extension cylinder and the external extension cylinder; the three-step sealing is formed by the sealing of the sealing ring on the internal extension cylinder, the sealing of the internal end surface of the circular trough and the sealing of the external extension cylinder, thus ensuring the drilling liquid to meet the requirement of the close loop circulation. The seat packer is used for realizing the quick sealing connection of the reserved slot of the waterproof conduit and the wellhead returning pipe; the device of the utility model has quick installation, improves the operation efficiency, effectively saves the drilling time, reduces the cost, effectively reduces the operation risky in-situ, and reduces the potential safety hazard.