A multi-transmitting boundary is a local artificial boundary widely used for numerically simulating seismic site effects. However, similar to other artificial boundaries, the multi-transmitting boundary has instability issue in numerical simulation. Based on the concept of multi-directional transmitting formula, a strategy for eliminating the high-frequency instability of the transmitting boundary is studied and a measure is proposed using a neighbour node of a boundary node to realize smoothing filtering. The proposed measure is verified through numerical analysis. The smoothing coefficient chosen for this measure provides a reference for deriving the coefficient of multidirectional transmitting formula in the time domain.