A novel fibre surface plasmon resonance (SPR) sensor fabricated by a silver mirror reaction is first proposed and demonstrated in this paper. The experimental results showed that the silver film characteristics of the fibre SPR sensing probe are affected by the concentration of silver ammonia solution, and the relations between the concentration of silver ammonia solution and properties of the sensors have been obtained, which is in accordance with the optimal parameters of fibre and silver film through a theoretical simulation. Firstly, a theoretical model of a silver film-based fibre SPR sensing mechanism has been built up. Then the numerical simulations towards the influence of sensing structure, thickness and sensing length of metal films on the sensing system sensitivity have been performed. Finally, the optimal structure parameters of the sensor are obtained. The results show that this fibre SPR sensing system provides a promising platform for sodium chloride solution concentration measurement with a concentration sensitivity of 710.4 nm/%.