The invention relates to a sustained-release suspension for treating glaucoma and a preparation method thereof, effectively solving the problem of low bioavailability in the prior art. According to the technical scheme, the sustained-release suspension comprises the following components in percentage by weight: 1.0-5.0 percent of timolol maleate, 1.0-5.0 percent of IRP-69 cation exchange resin, 0.1-10 percent of a micro-capsule coating material, 0.1-10 percent of plasticizer, 3-10 percent of impregnant, 0.1-20 percent of suspending agent, 0.1-10 percent of osmotic pressure regulator, 0.001-0.02 percent of preservative, 0.1-10 percent of pH regulator, 1.0-20 percent of sorbitan monooleate, 5-50 percent of liquid paraffin, 1.0-50 percent of micro-capsule coating material solvent, and 30-70 percent of super-pure water. The sustained-release suspension is safe in administration and definite in curative effect, does not have irritation, can remarkably improve the bioavailability of timolol maleate, and is an innovation in the field of medicine for treating glaucoma.