This paper presents a new underwater double piezoelectric energy harvesters (DPEH) system. Two harvesters having same parameters are arranged in series. Three groups of experiments were carried out using the DPEH-101 (diameter 10 mm, specific gravity 1.18), DPEH-102 (diameter 10 mm, specific gravity 2.7), and DPEH-151 (diameter 15 mm, specific gravity 1.18) systems separately. Output characteristics of the above systems at different spacing distances between two harvesters were studied and compared with that of the single piezoelectric energy harvester (SPEH) system. Both the output voltage and the effective output range of flow speed (EORFS) of the DPEH system were significantly improved, compared to the SPEH system. When the spacing distance is 2D, the outputs of the harvesters facing the flow of the DPEH-101, DPEH-102, and DPEH-151 systems are 27%, 30%, and 5% higher than that of the SPEH system, respectively. The outputs of the harvesters in the wake of the DPEH-101, DPEH-102, and DPEH-151 systems increase by up to 67%, 60%, and 99%, respectively. This study provides a new idea to present promise in amplifying the power and broadening the frequency range for underwater energy harvesters, and contributes to the fluid dynamic structural design for new micro unmanned underwater vehicles. (C) 2019 Elsevier Ltd. All rights reserved.