As one of the most common mountain hazards, the collapse of dangerous rock at high altitude with freezing temperature in Sichuan-Tibet Mountains presents a serious threat to the driver traffic safety and people survival. However，there are less effective measures that can be used to diagnosis and treat these frigid danger rocks just for its mechanism is not clear at present. Thus from the true structure of danger rock and based on the rock fracture mechanics，the expression to calculate the expansive force of crack ice is developed. Then by analyzing the condition and process of crack propagation，the instability mechanism of dangerous rock under ice expansive force is studied clearly. Furthermore，the calculation formula for critical temperature rise and propagation length are also suggested. At last，an example is given to prove the theory. The result shows the cracks on danger rock can propagate and develop when the temperature rises higher than the critical value, which result in the collapse occasion at last.
