Many efforts have been made to develop time division multiple access (TDMA) slots allocation in a multi-hop converge-cast wireless sensor network (WSN), however, most of them either use complex algorithm or concern frames only without simultaneous transmission in a single slot. In this paper, we present a timeslot assignment scheme for cluster-tree-based TDMA WSN, covering three frequently used working modes in practical applications. The shortest frame formed can guarantee real-time communication and is also facilitated for message and slot integration, since timeslots allocated to a single node are continuous. During allocation processes, the algorithms are distributed and light-weighted. The experiment resulted from a WSN prototype system shows that our scheme can achieve a good reliability.