Magnetic responsive photonic crystals (MRPCs) stand for a new trend in the field of photonic crystals. Compared with normal responsive photonic crystals built by sensitive hydrogels, their intriguing properties, such as instant responsive behavior and more uniform diffractive color, are most attractive. The synthesis of magnetic nanocrystals for MRPCs requires a highly reliable and facile method to guarantee the monodispersibility and stability. Popularly used methods, especially those claimed as one-pot methods, were conducted in Teflon-lined autoclaves. However, several shortcomings, like expense of large scale equipment, safety issues with generating high pressures in reactions, and latent pollution issues of Teflon liners, have restricted the large scale application of autoclaves. Thus an alternative facile one-pot method for MRPCs has been developed in our work. It is simply conducted in glass containers with milder conditions such as ambient pressure, air atmosphere, and relatively low reaction temperature. Moreover, by fixing a relatively high stoichiometric ratio of alkali, the size of each unit can be readily controlled by the additional amount of water, as it is called the “rich-alkali-poor-water” (RAPW) method. The as synthesized nanocrystals are found to be useful in mapping unknown or irregular magnetic fields as one of their potential applications.