Abstract: In China, due to the influence of geological tectonics, the phenomenon of primary structural coal and tectonic coal stratification exists in many coal seams. The existence of tectonic coal leads to a sharp increase in the risk of composite coal seam outburst, which seriously restricts the safe production of coal mines. According to the gas diffusion state in soft and hard composite coal bodies, the mechanical instability law of composite coal with unsteady diffusion was simulated and analyzed. The calculation model of adsorbed gas expansion energy by unsteady diffusion was established, and the influence of steady and unsteady diffusion on the expansion energy of adsorbed gas was discussed. The control effect of adsorbed gas expansion energy on outburst was analyzed from the perspective of energy, and the disaster-causing mechanism of composite coal outburst instability by unsteady diffusion was elucidated. The results shown that, the existence of tectonic coals in the composite coal seams could increase the plastic damage degree to the adjacent primary structural coals, and improve the gas migration rate within the plastic zone of the primary structural coals, thus increasing the risk of coal and gas outbursts. The expansion energy of adsorbed gas in primary structural coal and tectonic coal under unsteady diffusion condition was 0.50 - 0.57 times and 3.36 - 3.38 times higher than that under steady diffusion conditions, respectively. The presence of tectonic coals in composite coal seams led to a huge gas concentration difference between the fissure and matrix systems within the primary structural coal, which resulted in the rapid desorption of adsorbed gas within the primary structural coal matrix, increasing the expansion energy of adsorbed gas in the adjacent primary coals, and thus significantly increased the outburst risk of composite coal seam.