Because of high concentrating ratios of Concentrated Solar Power Technologies (CSP), and maturity of industrial ammonia synthesis process, the high-temperature ammonia thermochemical energy storage based on CSP technology has a promising potential. As a prototype of a tubular ammonia reactor under un-uniform heat flux originating from a sunlight spot or line focusing, it is assumed that catalyst particles packed in it is isotropic, and the heat transfer and mass diffusion in the axial direction of the tube were ignored since the length-diameter aspect ratio of the tube is high. Consequently, employing the local thermal non-equilibrium method, a 3-dimensional model for an ammonia decomposition reaction in porous media was developed with semi-perimeter un-uniform heat flux. For further research, the diameter of catalyst particles, the porosity of packed particles and inflow modes were recognized as important factors. The results show that the particular axial or radial distributions of the porosity significantly affect the reaction characteristics. Especially, the distribution of radial linear reduction of porosity increases the ammonia conversion rate and reduced maximum wall temperature. At the same time, the smaller catalyst particles and the inflow mode of one-side in and one-side out all contribute to improving the performance of the tubular reactor. In general, the following measures can effectively improve the reactor performance: choose a smaller catalyst particle diameter, the porosity of catalyst particles packed decreases linearly along the radial direction from the center of the tube, and the inflow mode in the tube is one-side in and one-side out.