The Raman-scattering-based distributed optical fiber sensor (DOFS) can realize the continuous temperature detection in one-dimension space. It has good environmental adaptability, simple system structure, and high sensitivity. But there is a conflict between the spatial resolution and the signal sensitivity in the system, namely, when increasing pulse width of the probe pulse to improve the signal to noise ratio (SNR) of the detected signal, the spatial resolution is degraded simultaneously. Simplex code is used to overcome this conflict between the SNR and the spatial resolution. Using the theoretical analysis and numerical simulation, the Simplex code′s effect on Raman DOFS has been studied. In realizing a coding DOFS, there exits a problem of how to achieve high-power coding pump light, because the usual methods, such as pulsed-laser or internal modulation, can not be used. A method based on external modulation and optical amplifier is chosen, and a 4.7 km Simplex coding Raman DOFS experimental system has been set up. The results show that the 7-bit simplex code can improve the SNR of the detecting signal by around 1.6 times.