The invention provides an expanded flexible 
well cementing grout and a preparation method thereof. The 
grout comprises the following components in parts by weight: 100 parts of 
cement, 6-14 parts of 
toughening material, 3-80 parts of reinforcing material, 8-120 parts of density modifier, 0.5-2.5 parts of suspension stabilizer, 20-40 parts of high-temperature stabilizer, 35-100 parts of 
distilled water, 0.5-2 parts of dispersant, 0.5-4 parts of fluid loss agent, 0.3-4 parts of 
retarder, 0.1-2 parts of foam inhibitor and 0.1-2 parts of 
defoamer. The invention further provides a preparation method of the expanded flexible 
well cementing grout. The grout has good 
sedimentation stability; and the set 
cement has the characteristics of micro expansion, high strength and low elasticity modulus, and can avoid the phenomena of 
micro cracks, micro annuluses, even fracture and the like caused by 
cement sheaths in subsequent operation of gas storage wells, 
shale gas wells, dense oil-gas 
horizontal wells and the like, thus realizing good zonal isolation and providing a 
technical support for long-term, safe and effective operation of oil-gas wells.