The invention discloses an underground connector applied to a fuel gas transmission pipeline. The underground connector comprises a main pipe body, an inner supporting sleeve pipe and a pressing cover. The two ends of the main pipe body are each provided with threads, one end of the pressing cover is provided with threads and fixedly connected with the main pipe body through the threads, and the diameter of the other end of the pressing cover is smaller than that of the main pipe body; a clamping ring is arranged on the inner wall of the main pipe body and located on the inner wall of an inner main pipe body, the inner supporting sleeve pipe is located in the main pipe body, a predetermined gap is formed between the inner supporting sleeve pipe and the inner wall of the main pipe body, and a threaded hole used for glue pouring is formed in the main pipe body and sealed by a screw. The underground connector can be well connected with a steel pipeline and a plastics-steel conversion pipe fitting through the threads. Meanwhile, after the plastics-steel conversion pipe fitting is connected with the underground connector, the plastics-steel conversion pipe fitting is further fixed in a glue pouring mode. In addition, glue pouring protects the plastics-steel conversion pipe fitting, and the plastics-steel conversion pipe fitting is not prone to damage when being used in a bad environment.