The invention relates to a cement mixed component which is prepared from the following materials in parts by weight: 20 to 60 parts of oil well cement, 5 to 40 parts of metakaolin, 0 to 30 parts of micro-silicon, 20 to 50 parts of mineral waste residue, 10 to 30 parts of limestone and 5 to 15 parts of exciting agent. The cement mixed component disclosed by the invention can be applied to preparing low-density cement paste with the density in a range of 1.3 to 1.6g/cm<3>, and the prepared low-density cement paste has the advantages of having high early strength, good paste fluidity, adjustable thickening time, simple preparation method, low cost, excellent combination property and convenience in construction and being capable of meeting the requirement of well cementation construction of low-pressure easy-leak formation, especially marine well cementation construction.