Based on coupled-mode theory and mechanics of materials, the four-core fiber Bragg gratings (F-CFBG) transverse force sensing rules according to transverse load, direction and fiber outer diameter were studied. The F-CFBG reflection spectrum split into two main peaks under transverse load. The resonant wavelength changes and the space between those two peaks are proportional with the applied transverse force magnitude, but decrease as the transverse orientation increases. The ratio of the peak splitting between two cores at 90° separation only lie on the orientation of transverse. The numerical analysis of F-CFBG wavelength shift with different outer diameters was also presented respectively. On condition that normalized frequency keeps constant, the wavelength change and the space between those two peaks increase as the fiber outer diameter decreases, but the increase degree lie on the transverse orientation.