With increasing building energy consumption, the use of phase change materials (PCMs) to store energy becomes particularly important. However, relatively complex packaging technology, high cost and unstable thermal performance constrain the use of PCMs in the field of building energy efficiency. In this study, co-soluble hydrous salt/fly ash composite PCMs (FA composite PCMs) were prepared by straight dipping using the relatively inexpensive sodium sulfate dehydrate (Na2SO4 center dot 10H(2)O) as the primary phase change energy storage agent and solid waste fly ash (FA) as a carrier material. The FA composite PCMs were fabricated with an optimal mass ratio of PCMs: FA = 1.7:1. The PCM's chemical characteristics, morphology and thermal properties were systematically detected. It is shown that PCMs adsorbed to FA belong to physical adsorption according to FTIR and XRD analysis results. Confined in the microspores of fly ash, the phase separation phenomenon of FA composite PCMs is largely eliminated, and its undercooling has also been a certain degree of relief. In addition, the FA composite PCMs exhibit good thermal properties; for example, the latent heat reaches 106.9 J/g with a melting temperature of 25.3 degrees C and remains at 87.1 J/g even after 100 thermal cycles. Moreover, the FA composite PCMs exhibit excellent environmental sustainability evaluated using a new simple approach. Finally, no leakage was observed in the FA composite PCMs during the solid-liquid phase transition.