Large-scale uniform hierarchical broccoli-like zinc oxide (ZnO) superstructures assembled by nanorods were successfully synthesised via a facile solution method under mild conditions by using sodium carboxymethyl cellulose (NaCMC) as crystal growth modifiers. The morphology of the final products was simply tuned by adding different amounts of ammonia in the presence of NaCMC. In addition, the catalytic properties of as-prepared products were investigated. It was found that these ZnO superstructures have good abilities to promote the thermal decomposition of ammonium perchlorate.