Riprap protection is regarded as one of the most widely used methods for erosion protection of hydraulic foundations such as bridge piers in water. To accurately predict the scouring depth at the pile foundation after the implementation of riprap protection not only provides a calculation tool for the riprap amount for pile foundation maintenance engineering design， but also provides a means for the safety assessment of pile foundation for riprap protection design. This article experimentally explores the variation characteristics of the maximum scouring depth in the riprap layer at the pile with factors such as pile diameter， riprap particle size， flow depth， and approach flow velocity， respectively. The flume experiments show that in order to improve the stability of the riprap layer， the thickness of the riprap layer should beat least as large as the bedform height， and the range of the riprap layer should be at least larger than 5 times the pile diameter to avoid bedform destroy. Under these conditions， the larger the size of the riprap， the better the protective effect would be， and the larger the pile diameter， the greater the scour depth would be. Finally， a formula for calculating the maximum scouring depth after riprap protection is proposed， which would provide reference for future engineering design for riprap protection.