The invention discloses a universal joint for mud discharging pipe splicing. The universal joint for mud discharging pipe splicing comprises a lower ball body, a positioning ring, an upper ball body and an inner ball body. The universal joint for mud discharging pipe splicing is characterized in that a lower flange is installed at the bottom of the lower ball body, a plurality of lower rapid locks are installed on the outer side of the top of the lower ball body, a plurality of upper rapid locks corresponding to the lower rapid locks are installed on the outer side of the upper ball body, and the upper ball body and the lower ball body are assembled into a whole through the positioning ring and locked through the upper rapid locks and the lower rapid locks; and an upper flange is installed at the top of the inner ball body, wear-resistant pipes are installed in the inner ball body and the upper flange, and the outer surface of the inner ball body is freely rotatably embedded into a cavity which is enclosed by the upper ball body, the positioning ring and the lower ball body. The invention further discloses an assembling method of the universal joint. According to the universal joint, a main body is made of a steel structure, the universal joint has the advantages of being easy to mount, high in assembly precision, more stable in structure and long in service life, when the universal joint is deflected, better stability is achieved, and smooth progressing of the mud discharging process is ensured.