The invention provides a method for preparing methyltin maleate. The method includes the steps that methyl tin chloride and maleic acid serve as raw materials for preparing the methyltin maleate; a water-discarding solvent is added into wet-product methyl tin chloride and maleic acid in reacting, the mixture is subjected to a reflux reaction for discarding water at the temperature of 60 DEG C to 130 DEG C, the reaction is finished when no water is separated from an oil-water separator, and after aqueous phases are separated, a solid-phase product and a liquid-phase solvent are subjected to filter pressing and separated, the solid-phase product is washed and subjected to drying under reduced pressure through a fluidized-bed drier to obtain the finished methyl tin maleate powder, and the liquid-phase solvent can be recycled. The methyltin maleate prepared with the method can be used as a CPVC heat stabilizer; the raw materials are selected from industrial products, the reaction condition is mild and easy to control, the terminal point is easy to judge, the conversion rate is high, and the method has good industrial prospects.