The grouting methods have been widely adopted to improve the mechanical properties of weak rock masses in civil engineering, while the current study on grouting mechanism is still limited. Pressure monitoring is one of the most challenging problems when doing experimental tests on mockups and scaled models to study grouting mechanism. Based on a tunnel engineering case, this paper developed a grouting test to verify feasibility of the use of Fiber Bragg Grating (FBG) sensors in monitoring the pressure generated by grout injection, as well as to obtain the relationship between it and the external pipeline pressure. A total of 14 FBG sensors, mainly in the vault, spandrel and hance of the tunnel arch, were installed into the scaled model. Then, three grout injections were implemented to strengthen the weak rock mass. It could be measured that the average pressure increment and peak pressure increment of each sensor in the M1 grouting could reach 13 kPa~79 kPa and 25 kPa~89 kPa, respectively. The FBG sensors are therefore capable of monitoring the pressure caused by grouting. In addition, there was a pressure loss when the grout flowed from the pipes to the rock mass, which can reach 53.8%~82.9%, i.e. the pipeline pressure was larger than the actual pressure that drives grout diffusion, and this is mainly caused by the pipe friction and grout viscosity resistance. Therefore, a certain reduction coefficient should be taken when using external pipeline pressure to calculate the grout diffusion radius in engineering applications, otherwise the calculated values will be too large.