The present invention aims to provide a method for producing a chlorine-containing resin composition and a method for producing a chlorine-containing resin processed product, the methods being capable of providing a processed product that is excellent in thermal stability and heat resistance and that has various excellent properties derived from the chlorine-containing resin in an efficient, easy, simple, and high-yield manner, without degrading the appearance of the processed product. The present invention provides a method for producing a chlorine-containing resin composition, including: step (I) of dry-mixing a silicone oil and/or a liquid fatty acid having a C12 or higher carbon chain with a hydrotalcite powder to obtain a mixture; and step (II) of further mixing a chlorine-containing resin with the mixture.