The effects of R value, water content(wwater) and ambient humidity(RH) on the interfacial bonding strength between polyurethane(PU) foam and ballast were studied. The composition and morphology of PU foam were characterized by ATR FTIR and SEM. The results show that the bonding strength between the foam and the ballast increases with the increase of the R value. When the R value is more than 105, the bonding failure mode is cohesion failure. The foam density increases with less water content, showing better bonding strength. When the water content is less than 045%, the bonding failure mode is always the cohesion failure. Ambient humidity will affect the surface dryness of the ballast, thus affecting the bonding performance. There is no obvious change for the bonding performance when the ambient humidity does not exceed 80%. Taking into account the foam adhesive properties and deformation capacity, R value with the range of 105120 is appropriate, and the water content should be adjusted to control the foam with the density at 160kg/m3 or more. Moreover, the ambient humidity for actual construction environment should be less than 80%.