Volume-filling ratio of the working fluid has a predominant effect on the system performance of a closed two-phase solar water-heating (SWH) system. To study this effect, a prototype of a loop thermosyphon SWH system, which uses remolded flat-plate solar collector as the evaporator and the coil pipe in the water tank as the condenser, was set up. A set of long-term outdoor experiments under 10%, 20%, 30%, 50%, and 70% volume-filling ratios were conducted in this paper. R600a was used as working fluid. Loop thermosyphon solar collector thermal performance and system thermal performance under different volume-filling ratios, including the temperature distribution of loop thermosyphon evaporator, were presented. It is shown that the loop thermosyphon solar collector and the system had a better thermal performance than the conventional ones under 30% and 50% volume-filling ratio, and the loop thermosyphon evaporator had an even temperature distribution when the volume-filling ratio was higher than 30%. The optimum volume filing ratio lies in between 30% and 50% of the whole system volume.