Multicellular web-flange composite structures were assembled using glass fibre reinforced polymer (GFRP) square tubes as web sections and plates as face sheets, further with glass magnesium (GM) or gypsum plaster (GP) panels on the surface for fire protection. The structures were subjected to ISO 834 fire curve from underside where the GM or GP panels were installed and a constant load on top to introduce bending during fire exposure. Experimental results showed that the fire endurance times of the structures before failure were extended from 54 min without protective panels to 83 min or 103 min by a single layer of GP or GM panel. When double layers were used, the fire endurance time increased to 113 min for GP panels and 158 min for GM panels. Numerical modelling was further established to predict the temperature distribution in the specimens. Effects of the GM and GP layers on the thermal and mechanical performances of loaded specimens in fire could be clarified. The fire performance of GFRP multicellular web-flange composite structures enhanced by fire resistance panels could be well demonstrated experimentally and numerically.