The work aims to propose a new method of preparing fully dense metal deposit based on in-situ micro-forging assisted cold spray, so as to provide a low-cost preparation method for corrosion protection of magnesium alloy. Big sized shot-peening (SP) particles of 20vol.%~60 vol.% were mechanically mixed into aluminum powders as the spraying powder. During spraying, the deposited aluminum coating layer could be in-situ forged and densified by the big sized SP particles. Effect of in-situ forging strength on microstructure and corrosion resistance of Al coating was investigated. SEM was adopted to analyze the microstructure of the coating and electrochemical tests and long-term immersion tests were carried out to evaluate the corrosion resistance of the coating. As the SP strength improved, the density of metallic deposit gradually increased. When the SP particle content was higher than 40%, fully dense adherent Al coatings with porosity lower than 0.3% could be achieved by MF-CS technique. Electrochemical tests and 1000 h immersion test in NaCl solution showed that the magnesium alloy covered with fully dense Al coating presented the same corrosion resistance as the bulk Al and the corrosion rate decreased by over two orders when compared with that of unprotected magnesium alloy. After 1000 h salt spray, the interface between coating and substrate had no corrosion products, which showed that the coating could fully isolate the corrosion medium. Meanwhile, the micron level passivation film occurred on the fully dense coating and improved the corrosion resistance further. Through in-situ micro-forging assisted cold spray, fully dense Al corrosion protection coating can be obtained on the magnesium alloy at lower temperature and pressure. The technique is also expected to be used in other fields with density requirements for metallic deposits, such as preparation of high thermal conductivity and high electrical conductivity coatings, repair of metal components and manufacturing of added materials.