This paper proposes the critical conditions for a submerged ice block beneath an intact ice cover to become unstable, as a fundamental component of any numerical model to successfully predict the ice jam formation or the ice jam release events. The flume model experimental and numerical simulation methods are both applied to analyze the stability of submerged ice blocks. The flume model experiment is first conducted, and the experimental results indicate that the influencing factors of the stability of a submerged ice block include the relative length, the relative water depth and the relative width. It was shown that the effect of the relative width on the stability of submerged ice blocks was not well studied. Based on the experimental results, the k -ε turbulence model is applied to establish a 3-D numerical model for studying the pressure distribution beneath submerged ice blocks. The effects of the relative width on the Venturi pressure and the leading edge pressure are evaluated. Finally, according to the force balance equation and the moment balance equation, this paper proposes a computational formula for the sliding and underting criticalconditions of submerged ice blocks, and the results are in good agreement with the experimental results.