Abstract: The circadian clock which drives the generation of circadian rhythm, is an endogenous cellular timer that regulates various daily physiological and molecular processes in most eukaryotes and some prokaryotes. Nearly all of these organisms have evolved a circadian clock system, and fungi are no exception. The circadian clock plays an important role in regulating the growth phenotype, metabolic rhythm, and synchronization of environmental changes in fungi. Light is an important factor regulating the fungal circadian clock, mainly affecting the phenotype and periodical gene expression of fungal colonies. To understand the effects of light environmental parameters on the circadian clock of Penicillium expansum (a typical plant pathogen) and the structure, properties, and functions of its circadian clock proteins, and thereby analyze the relationships between its light-sensing system and biological effects, and to provide insights into the application of light regulation technology in the prevention and control of diseases in the storage and transportation of agricultural products. In this study, the morphology of P. expansum colonies under different light treatments was observed. Bioinformatics methods were used to predict and analyze the physicochemical properties, conserved domains, secondary/tertiary structures, phosphorylation sites, and protein-protein interactions of circadian clock proteins, as well as to perform multiple sequence alignment and construct a phylogenetic tree. The expression characteristics of genes were explored by using qPCR technology. The results showed that the P. expansum exhibited a concentric hyphal link rhythmic phenotype under blue light irradiation. PeWC-1, PeWC-2, and PeFRH genes were obtained by bioinformatics methods. The three proteins encoded by them were acidic and unstable hydrophilic proteins without transmembrane structure, with isoelectric points between 5.58 and 7.03. Their molecular weights were 97.15, 43.65 and 128.71 kDa, respectively. These three proteins have the same number and types of functional domains as those in the model fungus Neurospora crassa. Its secondary structure is mainly composed of α-helices, random coils, and extended chains. It has abundant phosphorylation sites and closely interacts with proteins in various metabolic pathways. The expression of the above three genes showed a rhythmic expression for nearly 24 h, and their expression was regulated by the feedback loop of the core oscillator, which also explained the reason why blue light treatment induced the spore zone phenotype of P. expansum with a period of approximately 24 h. These results suggested that the molecular mechanism of circadian clock regulation existed in P. expansum. This study will lay a foundation for further studies on the biological functions of the circadian clock in fungi using reverse genetics.