This paper presents an approximation method — Multiservice Overflow Approximation (MOA), to compute traffic loss in multi-tier hierarchical networks with multiservice overflows. With the MOA method, we first obtain the variances of multiservice non-Poisson overflows in each tier by a blocking probability matching approach, and then compute the call blocking probability of multiservice overflow traffic by a modified Fredericks & Hayward’s approximation. The results obtained by the MOA method are verified by simulations. Compared with an existing approximation method based on multi-dimensional Markov-Modulated Poisson Process analysis, the MOA method achieves an accurate estimation of overflow traffic loss at a much lower computational cost, particularly for high-load multi-tier hierarchical networks