Natural hydraulic lime (NHL) is a promising grouting material for stone heritage conservation, but is limited by low strength and high brittleness. 2-acrylamide-2-methylpropanesulfonic acid (AMPS) water-absorbing resin was used to modify NHL through one-step in situ polymerization, and the obtained AMPS/NHL composite material was characterized by XRD, FT-IR and SEM. Isothermal calorimeter was adopted to monitor the hydration heat releases of NHL before and after AMPS water-absorbing resin modification. The mechanical properties (flexural, compressive and bonding strength) of the cured slurry were tested using a universal testing machine. The results indicate that the acicular network structure generated by AMPS in NHL slurry can strengthen the internal connection of the slurry, improve the bonding effect, and significantly enhance the mechanical properties of the slurry. After curing for 28 days, the compressive strength of 3% AMPS modified slurry increased by 60.7 % to 5.37 MPa, with increase of 12.8 % and 32.5 % in bonding and bending strength, respectively.