The invention provides a solid polymer electrolyte, a preparation method thereof, a composite positive pole and a solid-state lithium-ion battery. The solid polymer electrolyte contains a network polymer formed through comb-shaped polymer crosslinking and lithium salt, a comb-shaped polymer includes a main chain and side chains, the main chain is polyethylene glycol bridged between silicon atoms and boron atoms, and the side chains are polyethylene glycol monomethyl ether connected to the silicon atoms or the boron atoms. A structure unit of the comb-shaped polymer has a structure shown as a formula I, wherein R is one of n-propyl triethoxy, vinyl triethoxy, allyl triethoxy, methyl triethoxy, chlorine triethoxy chloride, ethyl triethoxy and propyl triethoxy, m and n are independently 3-22, and x and y are independently 2-44. The solid polymer electrolyte is high in ionic conductivity, and the prepared lithium-ion battery is good in rate capability and high in low-temperature cycle performance and coulombic efficiency.