Abstract: The depth of socket piles is often uncertain in karst areas, and there is lack of reliable basis to evaluate the difficulty of pile formation and economical efficiency for pile foundation, which is a frequent issue encountered when an optimal scheme can only be produce among pile foundation, raft foundation and composite foundation. In this paper, the elevation at which a single pile starts to enter rock is defined as the elevation of the single pile entering rock. The average elevation of all single piles entering rock in a karst site is called the average elevation of pile entering rock in site (abbreviated as AER hereafter). The AER can be used to predict the variation range of the rock-entry depth and rock-socket thickness of all piles, which reflects the difficulty and technical economy of pile-forming in site. Taking the pile of the No.4 Building of the Jinsheng Plaza in Liuzhou as the engineering background and based on an existing analytical method of the entering-rock probability of piles, this paper establishes an analytical model for predicting the AER at karst building sites. All obligatory data for the model is abundant and inexpensive, which come from investigation and without additional work of test or survey. Two factors affecting the parameter AER, the dissolution feature and pile diameter, are reasonably considered in this model. The results show that the predicted ARE derivates from the measured value within 0.5 m and this precision satisfies the requirement of technical and economic demonstration of rock-socket piles, and can be used as the basis for optimizing foundation schemes and selecting pile-forming methods in karst areas.