Real-time performance and reliability are two critical indicators in an industrial wireless sensor network (IWSN). Several time-division multiple-address (TDMA)-based industrial standards such as WirelessHART and ISA100 are widely used in IWSNs. However, to simplify the analysis, standard TDMA supports only one or two slot types in each frame, and each slot is usually 10 ms, which severely limits transmissibility and real-time responses in TDMA-based IWSNs where the number of transmissions is large but the length of most packets is small. In this paper, we propose a TDMA frame containing slots of different lengths to address this issue. The key ideas are to waste fewer slot resources and achieve on-demand slot allocation. First, we study the matching problem in a TDMA frame; then, we propose two scheduling methods, the split scheduling algorithm (SSA)and the double plug-in algorithm (DPA), under our TDMA frame. Extensive simulations and real testbed results show that the proposed solution DPA can significantly improve network performance and reliability. Real-time comparisons with other existing scheduling schemes show that the proposed solution improves the acceptance ratio by 48.8% compared to the rate-monotonic scheme.