The invention discloses a synthesis method for monodisperse cationic polystyrene microballoon sphere with controllable grain diameter. Deionized water and an organic solvent are mixed into a 100ml threeneck flask according to proportions; under the protection of nitrogen, a certain amount of styrene, initiator and polymerizable-dispersible stabilizer are together added into the threeneck flask for mixing, stirring and heating with a stirring speed of 200 RPM (Revolution Per Minute) and reacting for 6-8h when the temperature is 70-90 DEG C. After the reaction is stopped, the mixture is naturally cooled to a room temperature and is centrifuged by a supercentrifuge; a supernatant liquid is abandoned, then sediments are washed by ethanol and dispersed ultrasonically for 5 minutes; then the process is repeated for 2-3 times to obtain sediments; the sediments are dried in vacuum under 50 DEG C, so that the monodisperse cationic polystyrene microballoon sphere is obtained with a grain diameter scope between 0.4 and 1.0 Mum. The synthesis method is simple and easy. The product of the synthesis method can be used in the fields such as protein carrier, coating additive and medical diagnosis, etc.