In order to analyze the failure mode and stability of block under complex surrounding rock，a numerical analysis method for block failure was proposed based on continuum mechanics. Firstly，based on the existing geologic plane modeling method of element reconstruction and node separation method，the grid-based block identification was carried out. Then，the expressions of contact force under various contact states were derived based on the explicit contact force method，and the contact analysis method of block and surrounding rock was proposed. Afterwards，the safety factor of block was calculated based on the strength reduction method. The numerical method can simultaneously calculate the deformation of surrounding rock and the deformation of block. A simple example of a double-plane sliding block also demonstrated that，under certain conditions，the numerical results are consistent with the results of rigid body limit equilibrium method. Finally，combined with a tunnel engineering across a fault，the stability of surrounding rock and blocks was analyzed. The results indicate that the numerical method effectively reflects the influences of stress and deformation form surrounding rock when evaluating the block stability. After failure of the key blocks，the potentially dangerous blocks occur in large deformation and the safety factor is low. This numerical method provides an effective analysis method for the stability evaluation of tunnel blocks under complex geological conditions.