In order to ensure the safe utilization of hydrogen energy, the explosion pressure behavior is extremely important to design chemical equipment and evaluate explosion accident consequence. This paper is aimed at establishing a theoretical method of predicting explosion pressure behavior in the confined chamber by considering flame instabilities. The tendency of flame wrinkling factor in the pressure-buildup stage is firstly evaluated using large eddy simulation and the compensation theory. The limiting value of flame wrinkling factor during entire explosion process is calculated using the fractal theory. Finally, the dynamic model of flame wrinkling factor is implemented into the smooth flame model. The results demonstrated that the flame wrinkling factor in the pressure-buildup stage almost increases linearly with time. The limiting value of flame wrinkling factor is 2.4649. The explosion pressure will be underestimated using the smooth flame model, and the calculated explosion pressure in the isothermal condition is smaller than that in the adiabatic condition. When the fully turbulent flame is considered, the explosion pressure will be overpredicted significantly. By changing the confined chamber size, the explosion pressure could be reproduced relatively satisfactorily when the flame wrinkling factor is assumed to increase exponentially. The explosion pressure prediction must consider the effect of adiabatic compression and flame instabilities on burning rate.