In recent years, some hollow core composite insulators, of which housings were made of liquid silicone rubber (LSR) of current and voltage transformers used in China Southern Power Grid have showed different levels of degradation and cracking phenomena. Based on SEM, FTIR, XPS and XRD methods, analyses have been conducted from several aspects such as organic functional groups concentration, microscopic appearance, element valence etc. It is found that organic groups (mainly Si-(CH 3 ) 2 ) on the side-chains of LSR are much more vulnerable than the backbone during the degradation process, thus Si-O/Si-C absorption peak ratio could be applied to characterize the degradation level of LSR. For severely aged LSR samples, the ratio is around 1.787-2.436. The valences of Si element and Si-O, Si-C functional group relative concentration of the aged LSR surface have changed and crystal structure is formed. Macro properties tests show that the surface of aged LSR is almost hydrophilic and tracking and erosion resistance is deteriorated remarkably. It is concluded that the crystallization caused by thermal/optical oxidation over crosslinking reaction is the main reason why degradation occurred.