In response to the challenges of slow development of cement slurry strength and long waiting time for cementing in deepwater/ultra-deepwater cementing operations at low temperature，nano calcium silicate hydrate（CSH）seeds can effectively promote cement hydration rate through crystal nucleation. Nevertheless，CSH tends to agglomerate and leads to uneven particle sizes. Additionally，the dispersants used during the preparation process can delay cement hydration，which does not meet the requirements for low-temperature cementing in deepwater/ultra-deepwater environments. For this reason，a gelatin grafted sulfonated acetone-formaldehyde condensate（SG）was prepared using gelatin，formaldehyde and acetone as raw materials. With SG as the polymer organic dispersion base solution，nano calcium silicate hydrate/gelatin grafted sulfonated acetone-formaldehyde seed crystals（SG-CSH）was prepared by the solution method. The microstructure，hydration exothermic law，hydration products and low-temperature early strength properties of SG-CSH were analyzed through laser particle size analyzer，transmission electron microscope，isothermal calorimeter and X-ray diffractometer，etc. The results showed that SG had excellent intercalation and dispersion ability，which could form a low calcium-silicon ratio（1.0），narrow particle size distribution（3—100 nm），good dispersibility，stable and precipitation free long-term storage seeds with CSH. After adding SG-CSH，more calcium hydroxide and perovskite were generated in the early stage of hydration reaction，accelerating the cement hydration rate. At the same time，the results of cement slurry thickening experiment and compressive strength testing showed that compared with the blank experiment， the thickening time of cement slurry was significantly shortened after adding SG-CSH. The thickening time of cement slurry at 30 ℃ was shortened from 210 minutes to 125 minutes. The compressive strength of cement slurry containing 4% SG-CSH increased by 1.9，1.3 and 5.0 MPa after initial setting for 1 day at 10，15，and 30 ℃，respectively，demonstrating excellent low-temperature early strength performance.