In this paper, we propose an efficient harmonic vector excitation coder (HVXC) speech encoder by using the framework of multi-stage voiced/unvoiced (V/UV) detection algorithm. With introducing the pre-stages V/UV decision, we embed the decision tree classifiers with coding procedure. Consequently, the first two stages of V/UV detection do not increase any extra computation to decide unvoiced speech in advance such that the modified HVXC encoder can adaptively adjust the flow of encoder functions to save the computation