Developing solid luminescent materials with a unity quantum yield and tunable emission color is promising, although it is still a difficult task. A straightforward heat‐treatment method has been developed to load 3,4,9,10‐perylenetetracarboxylic dianhydride (PTCDA) into the matrix of boric acid (BA) to produce powders with a near‐unity quantum yield and tunable emission color from yellow to green. Our results suggest that the emission of the powders originates from PTCDA, and the tunability of the emission color is caused by the hydrolysis of PTCDA in the alkaline environment. The near‐unity quantum yield is attributed to the BA matrix, which confines PTCDA. In addition, the powder also shows excellent thermal stability that allows its application in light‐emitting diodes. The above results are important for the development of solid‐state luminescent materials for various applications, and also provide a clue for studying the emission properties of luminescent materials. A straightforward heat‐treatment strategy is proposed to load 3,4,9,10‐perylenetetracarboxylic dianhydride into the matrix of boric acid to produce powders with a near‐unity quantum yield and tunable emission color from yellow to green. The confinement effect of boric acid is responsible for the high emission efficiency of products.