To examine the influence of slope angles and horizontal loading angles on the foundation proportional coefficient m in slope grounds，model tests of in-slope steel piles were designed and implemented，and the m-values under different slope angles and horizontal loading angles were then obtained.Model tests revealed that the m-value decreased nonlinearly with the increase of the slope angle α，horizontal loading angle β and lateral displacement x of piles at the ground surface.The m-value almost kept the same when the value of x was more than 10 mm.Moreover，the relationship among m-value and slope angle α and horizontal loading angle β was established.The contrastive analysis showed that the theoretical calculation result under the consideration of the slope space effect when determining the m-value was close to the test result，and the maximum error was no more than 10%.It is suggested to adopt the m-value when the value of x is 10 mm for the design of in-slope piles.