Literature has shown that the interface circuit plays an important role in a piezoelectric energy harvesting (PEH) system. By referring to the general model of synchronized multiple bias-flip (SMBF) and the recent implementation of parallel synchronized triple bias-flip (P-S3BF), this paper introduces a new implementation called parallel synchronized septuple bias-flip (P-S7BF) for further enhancing the PEH performance. By sophisticatedly designing the current steering network, P-S7BF realizes seven self-adaptive voltage bias-flip actions at every synchronized instant, such that it can further increase the net harvested power under the compromise of larger power extraction and smaller dissipation in power conditioning. The steady-state operation of P-S7BF is discussed in detail. Experiments are carried out on a prototyped piezoelectric structure under the same harmonic base vibration. The experimental results show that the ratios among the maximum harvested powers by using P-S7BF, P-S3BF, P-SSHI (parallel synchronized switch harvesting on inductor), and SEH (standard energy harvesting bridge rectifier) are 1.75 : 1.56 : 1.45 : 1.0, which validate the advantage of P-S7BF over the state-of-the-art solutions.