Remote optically pumped amplification technology is widely used in ultra-long span optical transmission system. Based on designed scheme of remote pump technology of the ultra-long span optical transmission system, the relation between gain and noise figure with optical signal-to-noise ratio (OSNR) of system is obtained, the structure design of remote gain unit (RGU)from the choice of erbium-doped optical fiber, mirror used, isolator position design is theoretically analyzed and experimentally researched, comparing gain and noise figure of remote gain unit at different schemes. Through the optimization of optical design, the optimal optical structure of remote gain unit is proposed under the conditions of different pump powers and the signal powers. The results show that using low-concentration HE980 erbium-doped fiber, high-reflectivity and low-insertion loss mirror and suitable isolator position, the gain and noise figure of remote gain unit can be obviously improved.