In this study, the effect of polymer decomposition temperature on the preparation of boron carbon oxynitride (BCNO) phosphors is investigated with the aim of obtaining materials with enhanced photoluminescence properties. Three types of polymers, including polyethyleneimine (PEI), polyallylamine (PAA), and tetraethylene glycol (TEG) are used as carbon sources for the formation of BCNO phosphors. PEI is found to have the highest internal quantum efficiency (IQE) of the three polymers because of its optimum thermal decomposition temperature and high exothermic energy during BCNO formation. The IQE of the BCNO phosphors prepared with PEI is 50%, representing a 130% increase over the value observed when TEG was used as the carbon source. In addition, the emission band of the BCNO phosphors can be tuned from 380 to 490 nm by varying the reaction temperature and polymer concentrations.