The invention relates to a determining method of an axial safety factor in road speed limit. The method comprises the following steps of: collecting fundamental data namely design parameters of road longitudinal gradients, and the like; calculating an axial highest driving speed; pilot-calculating an axial safety factor: firstly, supposing a limit speed to be 120km/h and calculating a stopping sight distance required by the limit speed; then calculating a redundant safety distance and allowable response delay time; next, checking the allowable response delay time, and if the allowable response delay time is longer than or equal to 1 second, ending the pilot calculation; if the allowable response delay time is shorter than 1 second, reducing a speed limit value by 10km/h and renewedly calculating the allowable response delay time until the allowable response delay time is longer than or equal to 1 second, and then ending the pilot calculation, wherein the specific value of the axial highest driving speed and the limit speed at the moment is the axial safety factor meeting the requirement. The speed limit value determined by the method reflects the quality of driving conditions, reserves a safety space, and is easy to observe by drivers.