Space time coding in the case where the channel state information is unknown has received only limited attention, although a signaling scheme called unitary space-time modulation (USTM) has already been proposed. An important reason for the impracticability of USTM is its too large calculation amount when meeting with a large size constellation. We propose a size-reduced method for constellation of unitary space-time signals for multiple-antenna communication system. Resorting to signal matrix level interleaving, we give a pseudo-random fading effect for the receiver to meet the channel requirement in theory, provided that the coherence time is a composite number. Simulation results show that compared with existing almost unachievable schemes, our method can greatly reduce the storage and complexity of calculation only with a slight punishment in theory.