The elemental rainfall-runoff theories of fields and grasslands were analyzed

and a method calculating elemental average infiltration ratio based on the data of rainfall-runoff was also given. This method surmounts the intrinsic shortcomings of one-dimensional experiment of vertical infiltration determining soil infiltration on slopes

and provides a convenient means for studying water interaction in slope land. The calculating results showed that elemental average infiltration ratio decreases with the increase of slope degrees.