The timber-concrete composite (TCC) slabs have become a preferred choice of floor systems in modern multi story timber buildings. This TCC slab consisted of timber and a concrete slab which were commonly connected together with inclined self-tapping screws (STSs). To more accurately predict the fire performance of TCC slabs, the mechanical behavior of TCC connections under high temperature was investigated by numerical simulation in this study. The interface slip of TCC connections was simulated by a proposed Finite Element (FE) model at room temperature, and different diameter and penetration length screws were considered. The effectiveness of this FE model was validated by comparing with the existing experimental results. Furthermore, the sequentially coupling thermal stress analyses of this model were conducted, and the relationship between the reduction coefficient of connection performance and the effective penetration length of screws was summarized. This study gave the fitting expressions for the reduction coefficient of slip modulus and joint strength. Finally, the numerical investigations of the fire performance of TCC slabs considering the char fall-off of Cross Laminated Timber (CLT) were performed to verify the effectiveness of the proposed reduction law. Comparing the fire-resistance time with experimental results showed deviation of the proposed model was -14.02%.