As the core of all-optical integrated photonic devices, semiconductor optical amplifiers (SOAs) can be widely used in all-optical communication and fiber sensing systems. It is noteworthy that material gain transparency of SOAs reveals the transition point of slow light and fast light, as well as the starting point of the gain in the signal propagation. Therefore, it is of great importance to measure input current corresponding to the gain transparency of SOAs. In this paper, a method for measuring the gain transparency injection current of SOAs was proposed. According to the characteristic that the output power of SOA is independent of the polarization of incident light when the material gain is transparent, the injection current of SOA is measured under different input optical power and the polarization state of incident light affects the least on the output power. Using the proposed method, the gain transparent current of the SOA is accurately measured as 155mA under at a given wavelength.