The present invention relates to a method for preparing a weather resistant reinforced acrylate based resin. More specifically, an activator-free conversion rate improver is included in an acrylate-styrene based graft copolymer, thereby improving the haze value, light transmittance, and yellow index when mixed with an acrylate-styrene based non-graft copolymer, and securing the weather resistance. In particular, according to the present invention, a weather resistant reinforced thermoplastic resin having excellent colors which are close to natural colors can be provided even without using sodium formaldehyde sulfoxylate (SFS) and iron ions.