A Fabry-Perot interferometer (FPI) cuboid cavity across the fibre for strain force sensing with high sensitivity is proposed and investigated. Theoretical simulations and calculation results based on the method of static mechanics analysis showed that the sensitivity of an FPI cuboid cavity across the fibre with a smaller cavity length is better than that of FPI cavities with other shapes. Then two FPI cuboid cavities across the fibres with the sizes of 18 × 60 μm and 15 × 60 μm were fabricated using a femtosecond laser micromachine followed by fibre fusion. Their strain force sensitivities reached as high as 21.95 nm N-1 (18.9 pm/μ∊) and 26.10 nm N-1 (22.5 pm/μ∊). The experimental results agree well with the theoretical analysis and simulation. The FPI cuboid cavities across the fibres can be used as strain force sensors because of their high sensitivity, good temperature-insensitive characteristics, miniature size and simple fabrication.