MoSe2 ultrathin nanospheres with three-dimensional network structure (MSS) were prepared by improved solvothermal method. These MoSe2 nanospheres are only 10 nm in size and actually composed of ultra-thin MoSe2 nanosheets with a thickness of only 2-3 molecular layers. Compared with the MoSe2 nanosheets (6-8 molecular layer thicknesses) of the three-dimensional flower structure (MSF) prepared by ordinary hydrothermal method, the MSS are thinner resulting in higher specific surface area of 5 times than that of MSF, and the light absorption ability at all UV vis spectrum is stronger. The photocatalytic and photoelectrochemistry results show that the photocatalytic activity of MSS is 17 times that of the MSF, and the photoelectrochemical performance is twice. The results of electrochemical impedance spectroscopy and fluorescence spectroscopy confirmed that the MoSe2 ultra-thin nanospheres with three-dimensional network structure have lower internal resistance and higher carrier transport and separation efficiency. In the most important three aspects that determine the photoelectrochemical performance of photo catalyst: specific surface area, light absorption capacity, carrier transport and separation efficiency, MSS exceed MSF. This three-dimensional network nanospheres structure can improve the performance of MoSe2. This research successfully demonstrates the enhancement of the properties of MoSe2 two-dimensional materials through structural regulation. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.