The invention relates to an oil well cement suspending agent for well cementation and a preparation method thereof, and a cement mortar for well cementation, belonging to the field of oil exploration.A preparation method for the oil well cement suspending agent comprises the following steps: (1) subjecting silicon chloride to pyrohydrolysis in a gas mixture of air and hydrogen so as to produce asubstance A which has hydroxyl groups; (2) reacting the substance A with a substance B and introducing carboxyl groups into a main body so as to obtain a substance C; and (3) grinding the substance Cto obtain the suspending agent which has a particle size of 90 to 600 nm. The suspending agent can function as a suspended lightening admixture in low-density cement mortar and a suspended weighting admixture in high-density cement mortar so as to prevent high-density/low-density cement mortar from settling and improve system stability; meanwhile, the suspending agent does not lead to thickening of cement mortar in low-temperature cement mortar nor decompose in high-temperature cement mortar, so a system is ensured to have good suspensibility and rheological properties at high/low temperatureand safe well cementation is guaranteed.