A solar thermochemical reactor is a device utilizing concentrated solar energy to conduct hydrogen gas production by two-step water-splitting by dissociation of a reactive material, such as zinc oxide (ZnO). Reactor design, heat transfer, and reaction kinetics contribute a significant portion to the achievement of high solar-to-fuel conversion efficiency. In this work, an investigation of an indirect-cavity type reactor design performance has been conducted by numerical simulation method by coupling the computational model of fluid flow, energy equation, discrete ordinate radiation, and species transport. The reactor consisted of a windowed cavity reactor with an array of five tubes containing the flow of the reactive material. Dissociation of ZnO in a steady state condition of the reactor has been assessed under 1,500 sun heat flux from quartz window. A parametric study has been performed for a variation of the particle's mass flow rate, solar flux peak in, and reactor configuration. The cavity of the reactor was insulated by a ceramic and reflective material to reduce the conduction and radiation losses. Inert gas of Ar was injected into the tube as product carrier. Energy balance analysis and reactor efficiency calculation have been performed to analyze the reactor performance. The results showed that the thermal re-radiation through the window and thermal conduction through the cavity wall dominated the heat losses around 84 % in total. The best operating condition in this study was at a mass flow rate 0.05 g/s, peak-heat-in 2,000 kW/m2, and tubes configuration of the staggered-front dominant. Some recommendations to improve the research include changing the chemical reactant with other metal-oxide which has a lower reactivity, increasing the tubes number to absorb the solar irradiation, combining the metal-oxide decomposition with other processes which require less heat, and applying the special material in window side which can filter the high wavelength from going outside the reactor to decrease the re-radiation losses.