A method and an apparatus for casting prefabricated prestressed concrete products with a substantially horizontal slipform casting process, in which method reinforcement strands are stressed in a bundle on a casting bed (2) before the slipform casting is started, wherein the expected behavior of at least one measurable variable affecting the strand stressing process during the strand stressing process is predetermined, and the behavior of the at least one measurable variable is measured and compared to its predetermined expected behavior during the strand stressing process.