The 25-year-old was set to join the Russian Premier League giants this month in a reported 15million euro deal and put pen to paper on a four-year deal
"I want to thank the Spartak Moscow executives for their consideration, but I will remain in Italy and at Cagliari," Astori said on the club's official website.
"I want to give a sense of fulfilment to my career with the Rossoblu outfit.
"I believe in this club and in the project.
"I will try to do my best and have a starring role next season."
Astori, a product of Milan's youth academy, is expected to sign a two-year contract extension with the Sardinian outfit.
His current deal ends in June 2014.