The invention discloses a method for determining fusing speed of protection slag of a crystallizer. The method comprises the following steps: grinding a protection slag sample, using dextrin liquid for blending the protection slag to prepare a triangular pyramidal standard sample; using a rectangular high temperature porcelain pad, wherein a triangular prism groove is arranged at the high temperature porcelain pad, the bottom area of the triangular prism groove is same to the bottom area of the triangular pyramidal standard sample; placing the triangular pyramidal standard sample in the triangular prism groove, drying the high temperature porcelain pad and the triangular pyramidal standard sample; placing the high temperature porcelain pad and the triangular pyramidal standard sample in an ash melting point tester, using the ash melting point tester for determining the fusing speed, and determining the fusing speed by analyzing the time corresponding to the sample shape change characteristic during a heating process. The fusing speed of the protection slag on a molten steel surface of the crystallizer can be acutely determined, the test temperature is close to the production reality, the determination phenomenon is visual, and the reappearance is obviously increased.