The invention discloses well cementation cement paste with a high water-cement ratio, low cost and a low density. The well cementation cement paste is prepared from the following raw materials in parts by weight: 40 to 100 parts of oil well cement, 25 to 60 parts of coal ash, 30 to 40 parts of silica fume, 2 to 3 parts of an early strength agent, 2 to 8 parts of a ternary exciting agent, 3 to 3.5parts of a fluid loss agent, 0 to 0.2 part of a dispersing agent, 0.2 part of a defoaming agent and 180 to 272 parts of clean water, wherein the ternary exciting agent is a mixture of soda lime and magnesium sulfate, the mass ratio of the soda lime to the magnesium sulfate is 9 to 1, and the soda lime and the magnesium sulfate are both sieved by a 100-mesh sieve. Meanwhile, the invention further discloses a preparation method of the well cementation cement paste preparation method. The well cementation cement paste has low cost; after the ternary exciting agent is added, the compressive strength is improved, and a requirement of well cementation for the strength of low-density cement paste is met; furthermore, expansibility is improved, cement paste micro annular spaces caused by the inherent shrinkage character are reduced, and the risk of poor wellhole sealing safety is reduced.