The invention discloses a calculating method for the journey time on the basis of a soft set. The calculating method comprises the steps that a traffic condition data sheet of a certain road section is built, wherein the sheet mainly comprises factors of speed, flow, environment weather, emergency and time; the mutual relation among the factors is obtained, and a fuzzy approximation space (U, R) is built, wherein U represents a set constituted by all the factors influencing the journey time, and R represents the fuzzy relation among the factors in U; the fuzzy rough set ([Q with a bar below]Ii, [Q with a bar above]Ii) of the fuzzy approximation space (U, R) represents the traffic condition of the road section, the threshold value set, occurring to the road condition, of the road section represents a certain type of traffic state of the road section through the fuzzy set Ii on the set U, and then the fuzzy rough set ([Q with the bar below]Ii, [Q with the bar above]Ii) is obtained to build a soft fuzzy rough set; the relevancy of ([Q with the bar below](t), [Q with the bar above](t)) and ([q with a bar below](t), [q with a bar above](t)) is obtained through a matching function, and then the traffic state of the road section is judged according to the magnitude of the relevancy; and the journey time is mapped according to the traffic state of a road.