Global navigation satellite system-acoustic (GNSS-A) combined underwater positioning technique is widely applied in seafloor displacement monitoring and offshore exploration. The conventional GNSS-A positioning strategy is under the assumption of equal-precision sea-surface transducer's positions determined by GNSS positioning, which weakens the positioning accuracy of single seafloor transponder-equipped station. In this article, the extended joint adjustment (JA) of the measurements of the sea-surface transducers to seafloor transponders and ranging measurements of the transponder to transponder is proposed. First, we refine the transducer-to-transponder timing observation equation system by acoustic ray-tracing strategy to reduce the sound-speed-related errors. Second, we establish the mathematical model for extended JA with interstation ranging measurements for seafloor geodetic network (SGN) positioning. Finally, the efficacy of the proposed method is demonstrated both in simulations and in real measurement datasets. The experimental results show that the proposed method outperforms the traditional positioning methods, especially in the horizontal components.