In order to clarify the microscopic process of Metarhizium rileyi strain CDTLJ1 infestation in the Spodoptera frugiperda larvae and to elucidate the pathogenic mechanism of the strain, scanning electron microscope, fluorescence microscope and light microscope were used to observe the process of conidia attachment on the surface of the larvae after inoculation of the 3^rd instar larvae with the strain CDTLJ1, germination and penetration, and proliferation in the larvae body cavity. The results showed that the germination rate of conidia of strain CDTLJ1 was 80.36% in strumae surface topography, which was significantly higher than 72.44% in gentle surface topography (P < 0.01). Spores could form germ tubes and appressoria after germination, but most of them used germ tubes as infestation structure. Conidia germinated within 8 h after inoculation, with peak penetration into the integument of larvae at 16~24 h. Larvae moulted at 24~32 h after inoculation, and the moulted epidermis was covered with spores adhering to the surface of the protozoan. The earliest appearance of hyphae body observed in the larvae's body cavity was 48 h after inoculation. The insects died after 96 h of inoculation, Hyphae were visible in the electron microscopic field of view as it penetrated the integument and grew outward from the host at 2 h after death. The germination time of strain CDTLJ1 conidia on the isolated epidermis was 16 h, which was later than that on the epidermis of live larvae significantly.