Explores the mechanism of subgrade soil arching and establishes available equation of soil arching. This helps selecting reasonable distance between piles to ensure the quality and save the cost of construction. Uses varying distribution load, establishes computer model of soil arching, and provides empirical relation equation of diameter between top section and arch foot section. Calculates the stress of pile top with the established equation and contrasts it with measured data, the results show that the two are basically consistent and verifies feasibility of the improved model.