The utility model belongs to the field of manufacturing technologies of float glass and provides a protecting device preventing a melting furnace crown from expanding. The protecting device preventing the melting furnace crown from expanding comprises a stand column (8), a truncated-cone scroll coil spring (6), a screw (4) and ejection iron (10). The stand column (8) is located between the truncated-cone scroll coil spring (6) and a springer beam (11), and a gap is formed between the stand column (8) and the springer beam (11). A jackscrew frame (1) of a frame type structure is fixed to the stand column (8). The ejection iron (10) is arranged in an axial through hole of the jackscrew frame (1), one end of the ejection iron (10) stretches out of the jackscrew frame (11) and is fixedly connected with the truncated-cone scroll coil spring (6), and the other end of the ejection iron (10) stretches out of the jackscrew frame (1) and abuts against the springer beam (11). Through the protecting device preventing the melting furnace crown from expanding, not only can uniformity of overall pretightening force of the crown in a cold state be guaranteed, but also stress changes of the crown and stress uniformity of all points of the crown can be visually measured and calculated in a thermal state.