Abstract: This paper studies the stability and failure mode of cave-roof under pile foundation. It uses the finite element software DIANA's phase analysis to simulate the plastic deformation and displacement field characteristics of foundation soil in the process of karst cave expanding gradually. Also it sets up five different load levels in contrast. It takes the ultimate bearing capacity with no cave as a benchmark to analyze the coupled effects of cave development and load level on the stability of surrounding rock and summarizes cave failure mode and mechanism. The following conclusions are draw: cave development and pile load both exist critical safe value. When the load is small, cave plays a role of main influence to foundation. The development of cave unceasingly makes plastic strain expanding downward along pile. When load level is higher, the plastic strain of same cave developmental stage increases significantly, even within the safe cave development can lead to ahead of security phase and early damage. Finally this paper simulates practice mechanical characteristics of surrounding rock better as karst growing with time in the evolution process, which has important reference value for stability study of karst-roof.